The default state of the monovalve is open. When you pull the plug, you allow full coolant flow to the heater core. Possibly without the circulation pump, but it'll get pretty darn hot.

I didn't realize that you are a tech. I hope I didn't come across as condescending. Not that I would be to an anybody.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD