For the belt to be released and pulled out you need to hold it the correct way up, as if it was mounted in the car. This can be a real pain and it can take a long time before you get it in just the right spot so it allows the belt to be released. If/when it does come out make sure you secure it and don't let it retract until your finished cleaning them! (as mentioned above).
I read that there is a pendulum-like mechanism within the belt, which only allows the belt to be pulled out if it settles in the correct spot -aka when mounted at the correct angle within the car.
Hope this helps