if you watch logic (he spams this move), you can see that you have to wait (drop like an inch from the edge) for the move to be consistent. you are probably not going to do this move constistent if you IMMEDIATLY up b. + the immediate version can be a bit slower (from the part where you initiate the up b) . but it's still more usefull with invincibility frames from the edge.
the way I use instant return is: drop from the edge (no fastfall), immediatly press your control stick to the stage and aim slitghly under sthe stage, then press up b and hold forward. this has a higher succes rate + it is very easy to use out of a double jump (if you are below the stage and want to get on the stage as quickly as possible)
also some cool thing you can do: I've seen people edgecamping like this: drop edge, uair, double jump uair, grab edge. If you figure out the coreect spacing after the second uair you can instant return after it instead of grabbing the edge.
instant return is really a sweet move xD like if you want to punish someone who is really close to the edge you are hanging on. you instant tether attack him xD and if is hes low percent you can followup quite nicely, you can even buffer jump out of instant tether to footstool or arial people (only with yellows)
also usefull if people are shielding near the edge and they dont expect the move lol.
thank god you discovered it
edit: when you buffer a jump out of instant return sometimes you buffer a single jump (which you can double jump after) and sometimes you buffer an immediate double jump, can someone figure out why that is...probably has something to to with the place you land.
when you buffer a double jump you can return to the edge and keep using tether (even if you have used it 3 stimes) this means the game thinks you have "connected" with the ground. however when you buffer a single jump out of instant return you can't keep tethering after you have used it 3 times...
also the single jump can be buffered if you TAP up even when tap jump is turned off (you can basicly buffer it with any button like A, B or even C stick). the double jump has to be buffered with X or Y (button jump).
double edit (kinda passionate today xD): seems like the single jump is some sort of bufferd edgejump...only possible if you are really close to the edge (so dont press forward all the way or else you will do the double jump). this also explains the tether loss if the game things you just did a ledgejump.