I don't think it's that simple. If you are running at 75% brake force, you are just resisting the motor turning. Therefore, any induced (regenerated) voltage will not exceed the supply (battery) voltage and no charging current will flow. Remember, these motors are ~35% efficient (according to patrick IIRC) when braking vs ~90% when running. So it makes sense you'd have to have a motor with decent amount of kinetic energy behind it (vehicle weight at speed) and stop quickly to generate a visible amount of regen current.
Patrick does explain, in depth, how regen braking works:
http://www.rc-monster.com/forum/show...0&postcount=57