I was under the impression that handbrake mode just prevented Half Tracks from unwanted rotating. So if you were using a M3 as a clown car it wouldn't rotate to try and fire it's main gun unless told to (which would count as movement and the passengers would be less accurate)
Didn't know that. Yes, that's what it does, I just tried it.
The handbrake mode description makes it seem like it should be the same as the AA halftracks ("The vehicle will not respond to movement commands...")
For the 251 though I would really rather it not be selectable. It's not like troops inside can fire outside. I would rather it be a static reinforce bunker when I need it to be and not have to fiddle with infantry around it.