They don't bounce at all, the elastic cord holds them quite firmly.

My "tail" light sits inside the rear vent of my Citi. The cord in that case acts more like a tether. The "head" light sits on top of the helmet between two vents. It's held in place.

I'm quite fond of Guppies and Mutant Guppies. I've got, I think, 5 or 6 of them in total. (some on my helmet, some spares in various bike bags.)
Besides the guppies, I've also got a Blackburn head and tail light set on the bike, and a solid beam headlight under the flashing Blackburn on my bars so I can see the ground in the dark.