Ahh, so actually timing it worked for you? My understanding of plunging attacks was that as soon as you swing, you're locked in the animation until you hit the ground, but I guess I've never really fallen far enough while attacking to be able to draw any inferences. Anyway, that's why I tried to swing immediately. But I'm not sure my character swung at all.
Plunging attacks are pretty glitchy in this game though. They do fantastic damage - even from low heights - but there's a weird delay. You actually hit the ground before your weapon hits the enemy, it seems, after which they get hit for a ton of damage.