It's worth pointing out that under the 1.5 exe, this control is slightly bugged. You have to check/uncheck the box several times to be sure that you will get the drip-fed reinforcements rather than them being under AI control. In particular, it has been reported that if you view the compsition of one of the armies before the battle, then you will end up with AI controlled reinforcements, regardless of what you previously selected. So remember, immediately before you press to start the battle, check and uncheck that box 2 or 3 times and leave it clear.