Belt tensioner for the long belts. The wave shape is designed to minimize point stress as in other designs. Also this design will not loosen up over time. (working well for about a year now)Instructions:loosen pullies before installing, then realign the X and Y axis.The belts for the Y axis are a bit more difficult to install than the X axis. It's important to put the open side pointing inwards, otherwise they will run against the sliding blocks.