focus 1.8 tdci T belt?

replaced T belt and WP, started engine with cover off to make make sure all was well and engine runs fine, but noticed that the [pointer/tab on auto tensioner was moving back and forth. switched it off and rechecked timing and belt tension, started it again and everything was fine, but tensioner was moving again. is this normal? does it move with the belt to take up any whipping/slack when engine revs are increased.