I'm not sure what the keys are to making this work yet. I havent' tried it out with anybody who didn't expect it...so it may be an abject failure in non-compliant mode.
The key so far is that uke has to be off his heels. If he's balanced and grounded he doesn't go anywhere.
But yet - the dropping into the seisan position keeps you from bending at the waist - it keeps me from getting pulled over by uke's forward motion. And I think, if you're stronger than I am you can keep hold of Uke's arm(s) and have a little more control over them than what I did in the video. But again, as we weren't using mats, I didn't want to keep uke from receiving the fall.