goes back in history where it was purely started by some operators as a symbolic movement to indicate V1 had been attained and to avoid the temptation of a startled pilot to abort after V1 while his hand was still on the throttles.
Sounds like an entirely reasonable idea to me.
Rotate with both hands and after the gear is selected up place one hand back on throttles. Pretty simple, I would have thought. "Encourages" you to keep it in trim, too.