girls with light abs moving/waving fast

Nauli is what you refer to

If the muscles move like THIS ...

Or like THIS ...

Most especially like THIS:

It's DEFINITELY a nauli. Click on 'similar images' and see a bunch more of 'em.

You may even find the particular clip you seek.

And then there's THIS ...

I have a feeling he's talking less about nauli and more about belly flutters. It's something that's done in belly dance that looks like a very fast movement of the stomach.

^^^^ True enough -- but FBBs rarely do flutters, so I didn't think about it. (The exception was Theresa Hendricks, but she's not to be found here on GWM.)

However ... https://www.youtube.com/watch?v=NctNagkqpHs