When you are mounting a camera, the centre of gravity shifts. A balance rail is necessary to compensate for this. The balance rail is suitable for Swarovski ATS, STS,AT, ST, CTC, CTS telescopes.
When you use a digital camera with the scope, along with the proper digital camera adapter, a significant amount of weight is added to the rear of the rig, which in turn alters the prime location for the scope's tripod foot relative to the FH101 head.
This Balance Rail, with one portion attached to the FH101 head and another spot to attach to your scope's tripod screw and video register pin will move the base of the scope forward the correct amount to balance the fore-and-aft weight distribution of your scope / eyepiece / camera adapter / camera assembly.