[EricH]

It depends on motor type. Also, the term you're looking for is "backdrive".

Window motor: Backdrive protection is built in. It's got a worm gear in the gearbox, and worm gears really don't like backdriving. Can you do it? Depends on the worm gear, but you really have to work at it.

CIM: I'd recommend keeping 4 CIMs in the drive, and we only get 4 this year. (It is also possible to use 2 CIMs and 2 FP.)

CIM, FP, or Banebots: Start by setting the speed controller to "brake" mode. Beyond that, it's a question of gearing and what sort of brakes you're putting on. I know 330 didn't have much of a problem with an FP-stock gearbox-2-stage chain/sprocket reduction (not sure exactly how big) when that was used in 2005, 2007, and 2008. It could be backdriven, but you had to work at it a bit--say, pull down hard. Under power, it could lift a high schooler. You could also put a pin through a motion part into a solid piece of robot to act as a brake.

A few more specs on the arm might be nice, like how much torque you expect the arm itself to generate when straight out and not moving.