The FH101 Fluid Head is the best that's out there, remarkably smooth and lightweight without the need for maintenance.
But when you use a digital camera with the scope (along with the #49209, #49210 or other manufacturer's digital camera adapter), you add significant weight to the rear of the rig, altering the ideal location for the scope's tripod foot in relation to the FH101 head.
Thus the need for this Digiscoping Balance Rail Quick Release Plate, which has one portion to attach to the FH101 head and another spot to attach to your scope's tripod screw and video register pin. The base of the scope is moved forward, appropriately balancing the fore-and-aft weight distribution of your scope/eyepiece/camera adapter/camera assembly.
Redistributes weight of digiscoping assembly