On rest state the change over valve directs return flow from heater core to engine return line. (Webasto supply is from engine upper hose.) (Webasto - heater core - engine - Webasto in series) When change over valve is activated (pre-heating) the return flow from heater core is directed back to Webasto. (Circuit bypasses engine coolant flow.) (Webasto - heater core - Webasto in series)
https://www.realoem.com/bmw/enUS/sho...diagId=64_1411