Pretty much the most important mechanic to know and understand to be able to hustle effectively.

Update - Disable Auto Free Cancel in options. This was a mechanic added to make it so new players didnt instantly self destruct, but it'll be ultimately detrimental to learning the game (it was a dumb addition).

Underneath the Lock In and Hold button, there is a switch titled Free. When selecting certain attacks, this switch becomes clickable, which then spends a free cancel (underneath the health bar) in order to make this attack able to be cancelled. When your opponent next has a turn, if the current attack you selected has not finished, you will be able to cancel it early into any move or use Hold to continue it. This allows for feints, mindgames, and safety on attacks. However, in order to get your free cancels back, either player has to hit the opponent with an attack, clash attacks with the opponent, or land a blocked hit.

Warnings:
-If you choose to hold an attack after free cancelling it and the opponent chooses to block in the same turn, this will always parry your held attack, no matter frame or height
-If the opponent is not actionable, you will not cancel. This means you are still open to all counterhits and projectiles
-Obviously, being parried disables your free cancel
-If you run out of free cancels, using additional free cancels will cost you a bar of meter each time (the button will turn Pink to indicate this)

BLOCK UPDATE - I won't be going over the full nature of the block update and its strategy here, go figure that out yourself. However, a huge part of strategy that people miss is the benefit to using attacks without FC that leave the opponent with 1-2f time to act, known as a meaty. This forces the opponent to block the attack, usually giving you block advantage. A key example of this is Cowboys Vertical Cleave, a frame 9 attack that is +3 on block, perfectly timed to hit an opponent using getup in the same turn. Being advantaged on block is extremely powerful, giving you strong mix with grab and an easy way to land counterhits on the flip side along with large amounts of meter for landing a blockstring on your opponent. I may write a guide that goes more into depth on this.

On both sides of the controls, there is a large blue wheel with the text DI above it. This wheel allows you to indicate a direction to influence knockback against you, simulating joystick directional input in a fighting game. While you may see some importance in this, there is another very important fact: the longer a combo against you goes on, the more powerful your DI becomes. Without using DI, combos may go on far longer than they would have otherwise, especially if you mixup the opponent using your DI and are able to land or fly out of reach. Additionally, your opponent may mess up if you move yourself using the knockback from a projectile with a specific DI and completely miss an attack. To avoid doing this yourself, try to predict what the opponent may select for DI using the DI wheel on their side of the controls.

Additionally, various projectiles in this game can be partially influenced by DI. Ninja can hit their own caltrops/red shuriken to launch it controlled by their DI and gain an automatic free cancel in doing so that allows them to act recklessly in neutral, Cowboy bullets ricochet based on DI and can be freely redirected on connect with foresight or when you detonate your foresight, and Robots bomb is controllable by DI when hit by either player (balling gameplay).

You've likely already seen whiff cancel (green colored option) appear after using an attack. However, what isn't immediately clear is that whiff cancel consumes 3/4ths of your burst meter on use. Thus, whiff cancel should be saved for emergencies or for mixing up the opponent in specific situations only.

Specifically, using a whiff cancel at point blank will NOT safe you, jab will hit you no matter what. Whiff cancel is intended to be used at a slightly greater range to save yourself when needed.

If you don't already know, whiff cancel is available whenever the opponent has a turn but you are still in an attack state that was not free cancelled. It allows you to act as if you had free cancelled and cancel your current attack, however, you are only able to select attack options or doing nothing, thus, you cannot cancel into a block or a dodge.

Quickly wrote this down because I keep finding myself explaining it to new people. It would be nice if somebody made a higher quality guide than this with images and stuff but I'm just writing this down in like 5 minutes for no reason sorry.

(nvm, this guide was absurdly popular so I tweaked and updated stuff lol)