Before You Begin...

Enable "Hasty Mode" in the Gameplay Options menu. This makes some actions, such as swapping and Marbles' charged attack, occur faster.
It is strongly recommended to bind glide and groundpound to a separate button from jump or attack. Glide and groundpound can both be bound to the same button.
Enable Game Timer and/or Stage Timer, which are required for full-game runs and level runs respectively.
Adjust screen shake and vsync to your liking.
Check out the Assists + Modifiers options. There are many options here which are useful for practice.

Core Mechanics

Maximum Speed

Marbles (the duck) can attain top speed by using a spin-dash (charged attack) or by rolling (attacking) through an enemy. Marbles loses speed while flapping, but does not lose speed from the first few frames of flapping. Therefore, if you need to flap, it's best to do so in small increments and as late as possible.

Checkers (the turtle) can attain top speed using a dropdash (groundpound, dash, jump). Checkers loses speed after rolling an enemy; to regain max speed, it's necessary to perform another dropdash.

You can maintain top speed by continuously rolling and jumping. If your speed is slightly below the maximum, it will be brought back up to the maximum after rolling. You lose a lot of speed by walking along the ground or letting a roll end on the ground, so it's important to roll immediately when reaching the ground (you can buffer it), and jump out of a roll before it ends.

Rolling through an enemy generates 5 freeze frames which do not stop the in-game timer (and slows Checkers down), so it's generally best to outright avoid enemies when possible rather than rolling through them. Stomping enemies and rolling boxes/barrels generates 3 freeze frames.

It's not yet perfectly understood which character moves faster, but they are close to equal. It's thought that Checkers moves slightly faster after a dropdash but loses a small amount of speed each roll-jump, while Marbles does not lose speed. This, in addition to the fact that Checkers must stop to dropdash, means that Marbles is the optimal choice in most horizontal levels, but in some levels Checkers is competitive. Checkers is optimal in vertical levels due to the quad jump. Checkers also moves about 15% faster in water.

Midair Swap

You can swap midair as long as you are in an "attack" or "piggyback" state. This is possible after rolling off ledges, using Checkers' dash (before jumping), or at any time while piggybacked.

Quad Jump

It's possible to piggyback during a dash which enables Checkers to effectively jump 4 times. The inputs are:

Piggyback with Checkers in front, either on the ground or during a roll
Jump
Press attack to throw yourself upwards
Groundpound, dash
Piggyback again
Jump
Throw again

After the second piggyback, you can also swap to Marbles, jump, and throw the shell to instead get a midair shell jump. This goes slightly higher and puts Marbles in front, allowing you to flap.

(Midair) Shell Jump:

To initiate a midair shell jump, begin with top speed, piggyback, jump, and throw the shell near (but before) the apex of the jump while continuing to hold the jump button. Shell jumps off walls are also possible, as seen in Dire Dire Ducts, but these are typically not useful for a speedrun. The video below demonstrates how to time midair shell jumps.

Infinite Checkers Fly

After dropdashing once, Checkers becomes "exhausted" and begins flashing. While exhausted, Checkers cannot dash again and will instead do a "dash fail." With correct timing, these dash fails can be used to gain unlimited height or distance. To perform repeated dash fails, hold jump while alternating between attack and groundpound. The attack input can be buffered during the groundpound, but the groundpound input cannot be buffered. Although dash fails are quite slow, they are useful in a few specific places during the run, such as B.V. Broadcast Tower and Hue's Shade. They can also be useful during casual gameplay and for recovering after making mistakes.

Clips

There are four known ways to clip out of bounds with varying degrees of usefulness. Some walls - usually the "curvy cave walls" seen in stages such as Salamancer's Sanctum or Aucora's Abyss - have no collision inside and you can move freely. You will need to clip again to get back inbounds. Other types of walls - usually "rectilinear" walls like in Shrine of the Salamancer or The Pip Ship - have internal collision and will prevent you from moving and also push you down. In these types of walls, you can move backwards by swapping repeatedly; you will move further if you un-piggyback.

Wall Clip

To begin, stand next to a wall, piggyback and face away from the wall. To perform the clip, press swap and the direction towards the wall on the same frame. This move can be executed more easily by binding a "clip button" to both swap (tagteam) and right (assuming you want to clip into a wall on the right, which is the only direction you need to clip in Any%). You can also bind a button to both left and piggyback, making the clip easier to set up, but the "setup button" can only be used if there is a platform to stand on next to the wall you want to clip into. You also need to be standing still (not rolling) for the "setup button" to turn you left. If you fail the clip using the clip button, it usually means you were too far away from the wall. Wall clips are the most useful type of clip and it are used several times in Any%.

Bottom Corner Clip

Begin by standing on the ground next to a wall, with Marbles in front. Piggyback and face away from the wall. Jump and swap when you land, while Marbles is "squished." This type of clip is used in Temporal Railroad to get under the train.

Top Corner Clip

You can clip in any location where there is a ceiling connected to a wall. Piggyback and jump into the corner, then swap when you reach the corner. This type of clip is easiest with Checkers in front. This type of clip doesn't put you all the way into the wall, so another swap is required to get through: either maintain the piggyback and face the same way, or exit the piggyback and turn around before swapping to get in. This clip is pretty much entirely obsoleted by the wall clip, as that clip is easier to execute and only requires one swap.

Here is a video demonstrating each type of clip:

Sloped Speed Clip

It's also possible to clip through sloped one-way wooden platforms if you're going fast enough. The most common places to encounter this are Pips and Pits and Over the Raybow. Clipping is detrimental in both situations. It may be easier to avoid clipping with Marbles since the characters have different hitboxes, but this isn't confirmed.

Niche Mechanics

Input Buffering

"Buffering" refers to pressing a button early to make the action occur on the first possible frame. The "buffer window" refers to how early an input can be buffered and still received. You can buffer any input when entering a stage with an unlimited buffer window. The buffer window during normal gameplay is quite generous and most inputs such as rolling or jumping before landing can be buffered. You can buffer both dash and jump out of a groundpound (dropdash), but the buffer window is slightly smaller than the duration of the dropdash so you need to wait a few frames. You cannot buffer a midair piggyback after a dash, and swaps can never be buffered.

Autofire Cannons

You can hold jump to fire the auto-fire cannons about 22 frames (0.366 seconds) earlier than normal, which can add up across a chain of cannons.

For example, in B.V. Broadcast Tower, holding jump saves about 1.8 seconds in the beginning section over not holding jump.

Extending Rolls

The duration of a roll can be extended by rolling off a platform, rolling through an enemy or using a spin-dash (Marbles).

Higher Jump

Just like Mario, the heros of Windswept can gain extra height by jumping while moving fast. The difference is just a few pixels, but it can matter in a few specific locations.

Maximum Speed out of Horizontal Cannons

After getting fired out of a cannon horizontally you will immediately regain maximum speed with a roll. You do not need to spin-dash or dropdash.

Midair Spin-Dash

If Marbles is rolling, it's possible to charge and release a spin-dash in the air. This can be useful for instantly turning around, like in Brambles in the Breeze, or for getting the roll to cover extra distance, like in Bridge-Wheel Waterway. This move can be used repeatedly.

Holding Jump While Gliding

Holding jump while gliding actually makes you fall a tiny bit slower. This makes a difference in a number of places, including Smoky Squall where it causes you to rise slightly faster.

Camera Drift

If you run right at maximum speed for a long time your character will slowly move towards the right side of the screen, making it hard to see in front of you. This is typically only noticeable in very long horizontal levels such as 8-9 Cyclonic Skyway or 8-8 Dreadmaw's Dwelling.

Semisolid Groundpound/Dropdash

Using Checkers, if you go up through a one-way wooden platform and groundpound when you're just barely above it, you'll get a groundpound without any wind-up animation. This groundpound can be converted into a dropdash, allowing you to gain maximum horizontal speed immediately.

In levels that begin above one-way wooden platforms, including Hue's Shade, Thornado and Pips & Pits, this maneuver can be executed immediately upon spawning in at the start of the level. The easiest way to do this is to simply hold the groundpound button to buffer the move at the start of the level.

Snap to Ground

If Checkers groundpounds a few pixels above the ground, you will be brought down to the ground, even if a dash is buffered. This is generally undesired as it loses height and disrupts alignment, although it does refresh the groundpound. Checkers will also snap to thorns, taking damage.

Groundpound After Roll

If Checkers walks off a platform, there is a small delay before a groundpound is possible. This delay is not applied if Checkers rolls off the platform. This is useful in a few places, such as the Pip Ship.

Some Frame Data

This section catalogs the number of frames required for certain moves. f = frames, s = seconds (IGT), 60f = 1s.

Rolling through an enemy: 5f, 0.083s
Bouncing off an enemy: 3f, 0.050s
Rolling through a box or barrel: 3f, 0.050s
Spin-dash: 18f, 0.300s
Groundpound: 20f, 0.333s
Swap: 30f, 0.500s
Piggyback: 15f, 0.250s
Un-piggyback: 30f, 0.500s
Checkers up-throw: 16f, 0.266s
Marbles shell-throw: 16f, 0.266s
Buddy bubble: 30f, 0.500s
Damage: 48f, 0.800s
Death: 48f, 0.800s