I only use onehand or finger ;)
I always signal even if no one is around just because I want to be in the habit of doing it. I have a really hard time using my left arm for signaling...I even start to get off balance on my bike, but thought that was the "way" it had to be...so that's what I did. I'd much rather signal with my right arm.
I learned it this way:
Using your left arm...straight out means left
holding your hand up, forming an L, means turning right
hand down (reverse L) means slowing and/or stop.
Using your left arm...straight out means left
holding your hand up, forming an L, means turning right
hand down (reverse L) means slowing and/or stop.