I use a th400 that coan sells called a big dog with a reverse valve body. I have had thoughts of what may happen if I accidentally activated the brake during a run because I do not use a delay box to break the circuit after activation, so the whole run is with the switch hot on the wheel. Craig, at Coan told me that there valving set up ( and I am sure most others as well) the brake activation will dump the oil pressure to the forward clutch pack and torque converter while in third gear if accidental activation occurs. He said that if under full power, you will torch the forward clutch pack with the pressure loss that the brake activation will cause. I use to turn off the power switch between the 2/3 shift back when I ran 8.00 Et quarter mile times. When I started going faster I did not have the time to turn off the switch between gear changes. Steering was more important and that is why I asked Coan (what if)
You were asking about in the pit's in first gear, well the brake works in first so if you had enough rpm in first an activated it I believe you will know why the scatter shield on the top of the trans. It will explode when the gear set comes to a sudden stop with lots of rpms in first. At a very slow speed, the gears are not going fast enough to explode like it would at the end of the 60' under full throttle if you reactivated it in a run while still in first gear.
Bruce