A tracker needs nothing less than total body control. Total body control requires a strong core, fascial fitness, good upper/lower body coordination and stable tendons. The Tracker workout may not be the hardest in terms of difficulty but it does tackle all of this and because it is not super-hard, it does mean that you can do it more than just one day a week. Add Ec and the challenge does go up a little but it is still a very accessible workout.
Extra Credit: 1 minute rest between sets.