I think of it in terms of the flow of evidence. The host knows which door the car is behind. Therefore if he makes decisions based on that knowledge those deci

I think of it in terms of the flow of evidence. The host knows which door the car is behind. Therefore if he makes decisions based on that knowledge those decisions can give you information about the location of the car. If there is some action that he is more likely to perform when the car is behind door 1 than when it isn't, and he performs that action, then you should increase the probability you assign to the car being behind door 1. In particular if you've chosen door 2 then the host is more likely to reveal door 3 if the car is behind door 1 than if it isn't.