I think it is a vaccum switch, not positive though and there would have to be more to it than that anyway. I do know that it isn't computer controlled. The TH350 was used before computers were even put in the trucks.
on a 700 there is a vacuum switch that if the brakes are off and vacuum is right it send power to the TCC. If you hit the brakes it cut all power to the switch so it can't lock while hitting brakes. You might checkout bowtieoverdrives.com, I think they have some info on that regarding the 700, I don't know if it is the same for a lockup TH350