its because as you hit the ground with dair, 2 small hitboxes appear either side of him. these hitboxes are why you see dair failing to be l-cancelled on many mario vids. you need to hit l a fair amount later.
i just realised we are on different pages - you are talking about jumping out with a dair while their upB is charging yeah? that is very effective against falco. the limitation is that most of the time a spacie will be able to sideB onto the edge/ledge before we have a chance to get over there.
its difficult to setup - CPU's make it really easy but it can be difficult to get an opponent skilled in recovery into the right position. one way is to jab/Dtilt (down angled) them out of their recovery. the jab and the dtilt are pretty much exactly the same apart from damage i believe. no stun, just a little knock (dtilt ***** ganon/falcon comming from below)
when it comes to fox and falco
if they like to sideB onto/above stage, jab/dtilt
if they like to try and sweetspot with sideB, dilt
if they are above and away from the stage and are going to try and sweetspot upB (they do all the time) dtilt will 100% be in the way, because they are comming down, and the dtilt covers all angles apart from sweetspot sideB.
if they are directly infront of the edge, and are looking to travel in a straight line onto the ledge, you could dtilt. but seeing as they cant sweetspot from this angle (from my xp?), miles better off with a charged dsmash.
if they are below, cape. But if they are smart they will take advantage of the fact that travelling directly up turns them around, so you actually cape them into a stage grabbing position - time the cape late, then dsmash. You could also jab them out of it, or not do anything, and hope they didn't leave enough room to land on the stage after turning around.
these moves would setup an off-stage dair every time unless they tech, that position, them being directly below the ledge. i like doing this very much - seeing as they are charging upB under the ledge, it is a simple matter to walk off the edge, start a dair asap, then after that hits them, do a rising bair/uair. if they dont tech the stage, they are dead. you could also cape... but caping a top fox/falco hardly ever seems to work unless you go offstage and cape right infront of them (guarenteed kill)
i believe this is the setup you are talking about to dair someone offstage - below and away from the ledge. almost 100% of the time your best option is to get inbetween them and the ledge and cape.
the limitations... once they are charging an upB below the stage, it pretty much over. the reason that its better to dair instead of just falling off and bairing is cos dair makes it harder to tech, and the bair needs to be timed pretty well.
also dtilt (down angled throughout this post) has no stun - remeber this.
the first paragraph of this post, that effect makes falcon recovering from below an impossibility - it covers the angle of his crappy sweetspot. SH FF an offstage dair -> rising bair/uair (SH FF so you dont get hit by his upB). beautiful
