[Lauren Child]

This one is clearer, from "Manual of Recovery (c) Recovery Technique Part 2; Practical and Technical Considerations" 7th August 1945.

The only thing I'd add is that :
a) many people use both arms for Advance
b) note the main difference for Halt is the elbows out and hands above eye hieght, while Advance/Reverse are elbows close to the body with hands at or below eye hieght.
c) to indicate a neutral turn bump your fists together twice with elbows out, before indicating direction normally, but remember not all vehicles can neutral steer/pivot steer.
d) some folks use a bent arm and fist (a bit like half of a halt signal) to indicate a change in direction - some would say it's safer as there's less chance of your hand going into something, and you can be more expressive with the fist for a tighter turn. That's probably why most folks use two arms for the Advance signal, so it's not confused.

Hope that helps.