Cool gas defrost systems use cool gas(vapor) from the top of the receiver instead of hot gas(discharge). The defrost valve, which is piped into the top of the receiver, energizes sending cool gas through the suction line to the cases. There is a bypass with a check valve around the txv. The refrigerant returns to the liquid header through liquid line. There is a solenoid in the main liquid header that closes on defrost with a pressure differential check valve,usually 20psi, piped in parralel. A pressure differnce is created by the closing of the solenoid which allows the returning refrigerant to flow into the liquid header. If the pressure drop increases above 20psi the check valve will modulate to allow enough liquid through to feed the remaining cases. If you have a solenoid for each circuit there will be a bypass for each of those also. If there is heat reclaim it should shut off during defrost to keep enough heat in the receiver for proper defrost.