What are boost tabs???
A boost tab relieves the load the pilot would feel in a pure mechanical control system.
Look at this one on an Edge 540..
It's hinged to the elevator, but attached rigidly to the stabilizer. As the elevator moves (in this case) down, the tab moves up. As it's located on the trailing edge, it possesses a good "hinge moment" for its size, and permits the pilot to deflect the control stick (and therefore the surface) further than he would be able to unassisted.