If the taz is completely powered off (as in off for a few days so all caps should be discharged) and I manually slide the bed, the LCD flickers on and off a few times. Are the steppers acting as generators or something?
Yes, and you can damage the crap out of your Rambo board if you send too much power through them. Best practice is never move any stepper while it is plugged into the board, and if you absolutly have to and cannot disconnect, move it slowly to minimize the electrical current generated.
Seems there should be a diode on those lines, unless that would interfere with the operation of the motors?
A diode would have a voltage drop, which wouldn’tbe ideal for motor torque. Some sort of Mosfet might work as well with less drop. I don’t know enough about the electronics to know which one would work though. That and since it seems like a fairly inexpensive solution, why aren’t people doing that? there are alot of 3d printers out there that could apply to, and none of them have one. That makes me think there’s a good reason not to do it.