Like everything else on rollers standing should be done smoothly.
Big jumps like during a sprint seems to upset the balance (for me).

Riding no handed came gradually, first with arms hanging by my side to
equalize balance. Later I was able to ride no handed and reach for things
on counter next to me, drink etc.

I saw one guy at a demo ride up on a BMX type bike, bunny hop (sideways?)
onto the rollers and continue pedaling. I'd have been sprawled out on the floor.

marty