Here's a picture of the thermostat cut open. You can see the path through the thermostat is a bit convoluted. Picture credit goes to Grady Clay, RIP fine sir
And here's some additional details on how the thermostat works
http://forums.pelicanparts.com/porsche-911-technical-forum/927332-how-external-oil-thermostat-works-illustration.html
And thanks for the correction by 76FJ55 regarding the pressure at the dead end when the thermostat is closed/cold.
Also note that the t-stat can be in open mode all the time. They do sometimes stick closed or stick open. I had one stuck open on my '87 Carrera and I have to assume when the oil was pretty cold the bypass passage was being tested. I could hear the oil gurgling in the oil lines on the passenger side of the car after startup