well its spring loaded so it should move against the spring.

i did mine on my 2l mk1 focus 2 years ago and i'm pretty sure i got undernieth with the spanner and pulled it towards me (towards the front of the car) to release the tension on the belt. could have done with a longer spanner though as the spring is bloody strong and i struggled a bit!

have you looked in a hayned manual? pretty sure it tells you in there or you could google search.