Signals before the intersection not as you are turning is what I was taught. What I tend to do is use both types of signals. I feel it lets those around me know better, even if it just piques their awareness through multiple motions, also I change hands as I need to scrub speed for tight right hand turns.

If I am in traffic I will ride in the left of the lane so I can see all traffic, so they will see my left arm, not my right arm.

I always have both hands on the bar for turns, so I suspected whomever is worried about hand off while turning signaling is doing it wrong.