Dave W was thinking very hard :
I tried a trial run with the switch out in the open so I could see
what happens to the plunger. Right enough it popped up at the start,
but didn't pop down afterwards when the resistor cooled down. However
a further adjustment of the screw made it pop down again.

They use a wax capsule/ plunger system on some door interlocks. Yours
sounds like one of those. A resister heats up a wax capsule, the wax
expands and forces out a plunger. Plunger retracts slowly, once the
heat from the resistor is turned off - so there is always a few seconds
delay between pressing the start button and the actual start, plus a
delay after the machine has finished, before the door can be opened.