"How can you bait something without conditioning? Baits are based off of abusing what they believe will happen."
you just move in such a way as that your movement telegraphs some likely move of yours. if the opponent doesn't believe that your bait is real it is obviously advantageous to them. However, if they think that your movement is a bait but it ends up not being a bait they often lose stage control or get wrecked so they have a lot of desire to believe that you just came in with an AC nair and missed.
this and what mow was saying pretty much answers the question of "bait without conditioning". the flexibility (in both degree and speed) of movement in this game allows for someone to provoke actions/movements even without time to condition.
as for what AC nair is..it is godlike. I feel like ending the post at that, but basically you just sh nair rising and then fastfall at a proper point (I do it with a delay because i don't want my fall to be so fast that i occasionally don't AC. However, I believe if you time it perfectly you may not have to delay) so that you land right as the nair is ending. it suffers some weaknesses as an actual approach on fox because he's too freaking short, but i like to mix in actual nair approaches when the fox isn't taking the bait.
i think actual nair approaches are dangerous vs fox irrespective of baiting scenario. i mean based on what you've described there is a discernible difference between the "bait" nair and the non-bait/approach nair. once the fox recognizes this trick, why should he have to microspace his approach when he could just run away and laser? i mean look marth gets ***** every time he jumps vs fox without a setup LOL and this manuever appears to be (and experientially for me) is not even a setup. if this is confusing i guess what i want to say is "why should fox rely on attacking you straight up and approaching when he can just rely on laser laser counter hit repeat"?
A few notes: AC nair's fastfall time on a nair moving forward with full momentum vs the bait type nair is different..basically the fastfall needs to be delayed if you sh naired moving forward and then held backwards to DI away.
works the same way in brawl, not sure why. i think it's because the game registers your momentum going forward and does not allow you to change your trajectory until you've sufficiently completed your forward momentum.
Why does nair baiting work? the fundamental issue is that fox wants to run in and hit you but must respect your nair. if he gives you the space necessary to actually avoid getting hit by your aerials if you actually approached then he cannot also be at a spacing where he can run in and punish your nair if you fade back afterwards. If he is amazing at dashdancing he should be able to pinpoint millimeter turns into punishing the nair, but if he is doing this he is not really respecting the fact that you could move forward while doing aerials and you should probably just approach.
once again there seems to be an assumption that fox WANTS to run in and hit you but WHY he has to isn't particularly clear. IMO fox does not even want to run in and hit you. he wants to use his movement options and his ranged option (ie laser) to provoke a response from marth so he can microspace and win on prediction or counter hit and win on reaction.
now IN the case that fox approaches for whatever reason, the third sentence is accurate...until the fox decides to stop coming in. the reaction window is sufficient such that fox can change his decision to come in (although he has to be pretty good at moving and reacting...and he has to not be afraid of marth when he is acting second in a situation where marth is close).
as for fox dash dancing i don't believe that he has to respect marth's ability to come in doing aerials >_> marth doing an aerial does not sufficiently act as pressure vs fox given fox's DD + nair/bair (to a certain extent)/dash attack (to a certain extent).
Fox tends to get boxed out at the ledge and then wants to get back in, but if marth is camping dashdance into AC nair at the right range fox is required to dash forward, dash back to avoid the nair and then dash back in immediately (even then, 4 frames of lag is not enough to guarantee a punish). Moreover he can't know for certain whether you will fade back your nair or just come in so he is taking a risk by choosing to attempt to punish you out of his dashdance.
OK this is a situation where AC nair is particularly useful and renders quite a few of my points before moot. you should've mentioned this from the start!
marths often just go straight into pivot grab if they see the fox chasing after the nair because they aren't really in a real amount of lag.
then marths are stupid LOL they have stage + limited fox's movement, pressure should be the #1 goal here not a read that punishes overcommitment
Anyway this is just one more tool at marth's disposal. it's particularly good when fox is boxed out at the edge. I'm not sure how good it is, as sometimes when watching pp, i think that maybe if i just dashdanced perfectly all the time is better..but this is definitely a lot easier to do. and 4 frames of lag allows marth to dtilt to punish them from coming in even if they are at a proper spacing. So many guessing games from fox's perspective. He really needs a godlike dashdance game to bait out marth's options at all. And if you do happen to be facing a fox who has that kind of control of his dash and nair spacings...then you should probably just approach once you've conditioned him to respect your aerial game
this paragraph goes all over the place.
4 frames of lag means that fox needs to have tighter timings and gives marth more leeway to play around with timing BUT the low punish window/flexibility of the move does not actually do marth any favors if fox plays a more counterhit kind of style. thus ac nair is severely inferior to better dash dancing because dash dancing does not require the fox to be greedy. there is no guessing game from fox's perspective because he does not have to ever make a distinct commitment to "proactive" vs "reactive" style gameplay given his movement + options out of said movement.
edit2: don't do AC nairs too close or on a fox who is just standing out of range every time. adapt and actually go in. else you'll just get dash grab upthrown a million times. I remember i went through that phase..it's not pretty, but at least your smash DI on his upairs may get better
"actually go in" = surprise tactic.
given scenario: fox stands outside of marth's effective range
yomi1 = you pre-empt fox's action and go into him
yomi2 = fox understands you want to go into him and DDs (away first), intending on counter hitting your aerial
yomi 3 = you pre-empt his attempt to counter hit setup and give up stage to punish his landing
yomi 4 = fox understands that it's just a bait and stops
at yomi2 and yomi 4 you only have a slight advantage at best, no advantage at worst (best = dumb fox, worst = smart fox). fox can run away, always, unless he is at the corner.
tl,dr: ac nair bait is strictly inferior to better dash dancing and should only be utilized as a situational bait at a higher level rather than a go-to option when fox is cornered