FL/PE2 - where does the momentum come from?

Hello!

I'm struggling a little with FL/PE2, because I don't exactly know what muscles to activate in order to get the rocks going. My rocks become very inconsistent over the course of the workout consequently.

 

So, I was wondering, what movement exactly causes the rocking motion in the hollow body rocks?

I'm not too sure looking at the video, is it a little momentum from the legs and a little crunching motion from the abs? Or do the legs stay in place all the time?

 

Thanks in advance for a little input

Ideally it's only coming from 'pumping' with your abs. The arms and legs are merely acting as weights.

Thanks a lot, Cole!

I will focus on abs as the only moving force then!