Raykz does a great job explaining how this works
So now that we know how this works its looking pretty amazing, especially because it actually acts like a real spike and can cause even stage bounce. I guess the real question is how often and how easily can Sheik get this to work, can we do it on almost every dthrown into uair combo and if we could should we?
It's good to note that you can also perform it much easier by just performing the multi-hit aerial while slightly above the opponent and then just fast falling into them (Just how Sheik does it the second time in that video).
By the looks of it this only works by landing before Sheik's uair completes, meaning it probably won't happen at higher percents. Seems fairly practical at lower percents, though, when juggles won't send the opponent as high into the air.
Yes, you can't combo INTO it at high %s because of how high all of your moves will send the opponent, but you can still combo
out of it.
I've personally killed a Bowser Jr at 100-110% by using an Up-Air Fast Falled Up-Smash with Greninja. Bowser Jr full jumped and I caught him with my fast falled up air brought him with me towards the ground, he bounced right into the Up-Smash that I instantly performed as soon as I landed myself, basically performing a kill combo on him.
It's good to note that the bounce can be teched, but if you know your opponent will tech it then you can tech chase into another attack or grab.