some trans questions

hello yall, i have a th400 im having some work done on and wanted to ask a few questions. first, what is the point of a transbrake? my understanding is it lets the converter "flash" to its highest rpm? how is the transbrake activated? is the solenoid held down, and then i release it when its time to go?are they advantageous with a lower stall? say 2600 rpm? also what are the advantages of a reverse action valve body? im guessing it takes a reverse action shifter? ANY suggestions? thanks again everyone..jason