Quite a lot apparently. This is my supercharger belt on a run at North Weald sprint.

Not suprised at all. Timing belts do similar... which is quite scary to watch.

Do you have a sprung tensioner?

Not on the supercharger belt. The part of the belt that is moving is the 'slack' return part.

I was surprised how much they move after my first visit to a rolling road, Walked around the front of the engine and

Interesting that maximum deflection seems to be associated with a gear shift, is this the effect of torque reversal on the belt as a result of a throttle lift? Not sure I can visualise the physics but presumably a reduction in tension on the drive side must be transmitted to the slack side in some manner.

Its pretty scary seeing a chain move in mid engined chain driven application on the rolling road too...