Checked the 928 manual. There is a solenoid in the line from dist to WUR, with a return line, connected to a temp sensor on rear of head. Theory is that while cranking, if sensor says hot, open solenoid, dump pressure lower than usual for temperature, presumably to allow rich mixture, or to allow air plate to deflect more, its not really explained much, just describes how to check that solenoid doesnt leak.
hth
jp