Understanding the oil filter canister ..

[elwood]

Hey guys, can someone explain to me how the oil filter canister works ..
I mean, duh, it goes in the top under pressure, and out the bottom .
Problem is, I pulled the filter out of the canister on my 50 F1, and the canister was still half full of oil, I even took the canister off and held in my hand and the oil still didnt run out the bottom fitting, so I pored it out and cleaned the canister,
Is there a check ball in the tube or something Im missing ?
Thanks ..