I’m so sorry I took so long to respond back here but I haven’t had my computer in like forever. Anyway, to do the float grenade maneuver, the opponent must be in the corner and you (as Cable) must corner them.
Next, what you then do is super-jump straight up and move in the air closer to the wall to where you are above the opponent and then just start throwing the grenades. Once you see if go off the screen, throw another one and repeat this process indefinitely. You’ll notice him starting to float all the way down as you keep throwing them.
And yes, it can be cancelled into an AHVB. Hope this helps.