Here is timing advance explained by Pdelcast (Castle President) on RC-Monster:

Timing Advance --

Let's see. Timing advance is just like the timing on a 1:1 car -- In an internal combustion engine, it takes some finite amount of time for the flame front to burn through the gasoline/air mixture in the combustion chamber, so the spark plug is fired early -- to make sure that by the time the piston is at top dead center, the fuel is mostly burned, and cylinder pressure is near maximum -- making the most power.
In an electric motor, there is an effect called inductance -- inductance is a resistance to change in current in a circuit -- so current ramps up and down, and doesn't change instantaneously. At low timing, the controller is actually centering the current ramp on the switch of coils, so that the efficiency of the motor is highest. But-- it is also possible to advance the timing even more, which increases the amount of current drawn by the motor (and therefore torque) -- however, this also increases (significantly) the amount of current drawn by the motor when the rotor is in a position where it doesn't generate torque efficiently, lowering efficiency.

So timing is a trade-off of torque generation (power) and efficiency. Above a certain amount of timing advance, and the rotor actually starts generating REVERSE torque for a short period, and then efficiency drops very quickly.