I am sure that a few others here have seen this video. When you are at very high cadence you have to have your feet firmly attached to the pedals. Years ago I was in a spin class and we were doing high cadence sprints. The instructor was watching me spin as fast as I could. These bikes were very basic, no electronics. He came over to me and announced another sprint. He placed his hand where my knee would hit it with each rotation, and counted the hits over 15 seconds. Turns out I was turning at 210 rpm. He did this because my very heavy spin bike was actually moving around on the floor at my top rpm. I can't get much above 140rpm on rollers but even then, at those speeds, you have to have foot retention, whether it is toe clips with very tight straps, or clipless pedals. I have been riding with my feet firmly attached to my pedals for over 50 years, including a fair bit of road and criterium racing as well as thousands of spirited group rides. I have seen instances where someone's foot slipped off their pedal while pedalling hard out of the saddle, usually caused by a worn out cleat. Isn't pretty