When the unit goes into defrost the unit stops cooling. A door closes to keep warm air from entering the cargo area thats behind where the air ducts attach to the unit. Hot gas flows from the compressor to the evaporator coil melting the frost. It drains out of the unit on to the ground though the drain tubes. I pull the fittings off of the bottom of the drain tubes to unclog them. When i was a teen back in the 80s i read though maintenence manuals