The pressure in your expansion tank should be equal to the pressure in your water system as regulated by your pressure reducer. If you have the pressure set in your system for around 70 psi (which is fairly high) then the pressure will be equal throughout the system including the expansion tank when the water is turned off.
The expansion tank is there to protect any pvc piping in your home that doesn't tend to like to be put under the extra pressure surges you can get from waterhammer and pressure from your water heater as the water in the heater expands. The water needs someplace to go as it expands, so it goes into the expansion tank, where the air in the tank can compess. (water doesn't compress as easily as air).
Hopefully, if I got this one wrong, someone will correct me, but I'm pretty sure on this one.