we can beat fox/falco bair if we can djc nair so that we're fast falling with them, and are positioned slightly behind them, as illustrated:
backwards DJ will give you more momentum than forwards dj so its very easy to confuse the falco/fox with this odd micro spacing. usually the bairs you counter with this backwards djc nair are ones placed to beat our sh/fh fairs
if we hit fox/falco with this kind of nair, they will either have to be ready to tech due to the low kb, or will be put into that weird landing lag sort of thing if they're close enough to the ground but not high % enough to enter tumble.
in the case of the former, that either means you can techchase pretty easily with dash attack/grab, or dtilt them if they miss tech.
if they're too low to be put in tumble, and you get that weird laggy landing state, you can combo into ftilt, utilt or grab depending on their DI and whether you ACd the nair or not.
if they're super low percent, ftilt isnt reliable coz they can shine/grab i think (esp if they accidentally di down because they were holding it to input shine/fastfall), so just reset neutral as best you can or go for a read if you have picked upon their habits in this situation
you can uair straight after the nair if you ledge cancel, or (pivot) usmash if you teeter cancel. you should be able to do the same inputs (hold backwards, jump, up+a) and do either options without having to react to whether u fell off the platform or just teeter cancelled, so that's neat.
uair also works in this situation, the spacing is just very specific and requires a very quick djc+fast fall, so its easy to screw up but usually you get huge combos from it. i usually claw grip to input these kinds of uairs otherwise it feels like im doing fighting game inputs lmao
this only works because hitboxes extend in the direction opposite your movement (so the spacie bair will be less disjoint than normal, and ness' aerial will be more disjoint than normal). since falco/fox fall faster than we do they suffer more loss of disjoint, and since we fall slower we benefit from less gain of disjoint - this fall speed dynamic is a little important when understanding what kind of DJCs you can do in matchups. i've found that ness relies on using djc fast fall to extend his disjoint as much as possible to make landing hard for other characters, as it really lets you control the momentum in matchups by hitting characters up and then landing asap and regaining stage positioning