It's possible to back-air or up-air from ledge so fast that you're able to grab the ledge again mid-air after the aerial finishes.
This technique is pretty pointless; if you hit a recovering opponent with an aerial from ledge, you'll probably want to go back on-stage to continue the edgeguard, and if you whiff the aerial from ledge, you'll really want to go back on-stage to avoid getting reversal-edgeguarded.
It's also hard to do; both your double jump and aerial input have to be extremely quick or you won't be able to grab the ledge and will fall below it.