Care to elaborate? I haven't followed brawl's competitive scene very much
background info:~~~
Okay.
"Hit stun cancelling" as a mechanic is this:
Hit stun exists as a value determined by move knockback, it is of similar values to that in melee.
Tumble is an animation/state where if the knockback of a move is high enough, you'll enter into it.
1. The amount of hitstun a move can have in Brawl before entering tumble is 30 frames. If the move doesn't send you into tumble, you experience
REAL HIT STUN (cannot air dodge or attack, or jump, or special). Another point is that
there are no DI modifiers on pre-tumble moves - this is the reason chain grabs exist (Falco, Dedede, etc) as they do in Brawl. If this didn't exist, all cgs, bar maybe ICs would have escape options/etc.
This gives emphasis on Smash DI to avoid moves chaining that 'combo' for long periods of time (most top tiers have quite a few).
2. If a move's knockback will make you tumble things change. You may air dodge out of tumble at 15 frames of it, attack at 24 (both numbers are 'IIRC'), you have normal DI ability.
However you cannot jump or use specials. Your only options are a 40 or 50 frame air dodge [taking you up to 55 or 65 frames of you not having control of your character] or attacks (character based, but aerials are usually 30+ frames). Let me highlight
ONLY OPTIONS.
~~~~~
1. Characters have combo moves, but most start with too low knockback to actually be safe on hit,
its almost like normalising crouch cancelling. Most characters rely on free + guaranteed throw follows up to get characters to a percent at which their primary moves start to combo. This is usually around the 30-50% mark. So from mid percent to usually just before kill percent, most characters have low knockback growth or low ending lag moves that aren't suitable at 0% but give true hitstun (that can be managed to last longer due to the move queue being 9) for elongated periods. At high levels, with buffering, players are executing combos or "near perfect" frame traps from hitconfirms and the like.
2. The amount of options you have are still meager. If you airdodge you cannot fast fall, only drift, and it's a big commitment. There's no way to jump out of tumble. You have two options only, aerial or air dodge and both are commitments, the main thing though is that
when you're hit, the place where you travel to is mostly deterministic, and although you have the option to attack or airdodge a follow up, your positional placement is 'set in stone'. This is where frame traps, good positioning and timing come into effect on the player who landed a hit, if they can move (buffer the action) to get into a space where they can avoid some out of tumble attack and can punish air dodges they're more or less guaranteed another hit. And this is what you see in 'faster paced' high level matches of brawl (not denying that some characters/matchups are cursed to not really be able to speed things up with either mechanics, but its not universal).
So for most characters, their aerials eventually combo well, as do their dash attacks. Hitconfirms are within reaction speed and if you know your character/enemies DI habits, you can take hits and continue the "awful positioning" chain until you mess up/they outplay you. You're still basically in a **** situation you shouldn't get out of easily - its the other things from brawl (no DI on pre-tumble, floatyness, etc) that negatively impact the game.