I'll try to explain how this brake works for the people that don't know much of electrical things (like me). Correct me if I'm wrong, please.
This "brake" is something you can set in the robot's programming. What it does is the same as connecting both the leads of the motor, when you press a button.
When you spin a shaft attached to the motor with your hand, you can notice a voltage on its leads.
For example: if you turn it to the right, you will produce 5 Volts. But if you put 5 Volts on its leads, it will turn to the left. If you put -5 Volts between its leads it will turn to the right.
So when you turn the shaft with your hands you are producing the opposite voltage it needs to spin that way. If the leads are connected, there will be a "contradiction", resulting in no movement, because it always produce for itself the voltage to spin in the way it's NOT spinning.
Let's say that your robot has this electrical brake on and another robot tries to push it. They are pushing you to go backwards, making your wheels spin backwards (as if you were spinning the shaft with your hands). The motor will work as a "generator", but the voltage it produces will try to make it go forward. So theoretically your robot won't move! And it's easier than using file cards!!
Maybe not so efficient...
This can be easiliy tested with the fisher-price motor with its reduction box!! Try to spin the white round thing with your hand, and then connect the motor's leads and try again!
And it's incredibly fun (at the first time you do it) to connect the Fisher-Price motor's leads to another motor's leads and then spin the white plastic thing on the FP motor's transmission box. The other motor will start working, and if you don't know anything about electricals it will look like magic
I hope it was easy to understand (but probably it wasn't!).
There are so many words I don't know in English, so it takes me too long to explain something easy.