94 Ford Escort 1.9L SEFI Thermostat Housing Question

The thermostat housing on this car seems to have a bypass port that bypasses the thermostat. When I shake the housing, I can hear stuff rattling around in there, but I have no idea how it works or what it is supposed to do.

What does this "bypass" valve do? How does it work, how can I test it?

You can see it right here... his left hand is pinching the valve part on the housing:
https://www.youtube.com/watch?v=8sK1GMKPpD4#t=256

The interior of that thermostat housing has been a mystery to a lot of 2nd and 3rd generation Escort owners. It is almost certainly a bypass of some sort - so the head wont get too hot prior to the thermostat opening, but no one has sliced one open to have a look inside and scope out what it does. It should rattle when you shake it though. It may also serve the purpose of letting the air pockets move out of the head, after a refilling of coolant.

Until I have a thermostat housing that is unusably bad, I dont plan to cut one in half. So far I have 4 2nd gen Escorts, and the housings all seem to work fine.