These nifty little clips attach to your drive belt and help to maintain a constant tension.
Insufficient belt tension causes lost motion or hysteresis. The end result is that parts end up undersize and there is a loss of definition/resolution.
With these clips you can tell at a glance if there's the right tension in the belt.
Make sure you fit them on a section of belt that is not required to roll over a pulley or bearing.
They should be installed as shown in the photo so that the tension arms press on the back of the belt.