The problem with the "calf on" principal is that you learn to ride on the back of your leg instead of the INSIDE of your leg. You will have your heels digging into the horse and your leg will lose a lot of its sensitivity.
I teach my student to keep their lower legs forward by taking their hands away from them in two point. At the halt, they will maintain their two point with their arms out like an airplane until they learn to move their lower leg forward to act as a counterbalance to their upper body weight. At all times their toes must be forward, not out.
Then, when they can maintain their two point at halt with no effort, they go to the walk on the longeline. When they can maintain their position without effort, they move to trot...then canter. This takes a LOT of strength and you will feel the back of your leg burn as it stretches. Then as someone said above, they will go through jump grids with their arms out until they come out of the grid.
This will build strength, teach you to balance without using your hands/crest release to prop yourself up, and teach you to ride on the INSIDE of your lower leg, not back of your leg.
Calf graspers are no better than knee graspers, IMO. For one thing, you will never be able to be trusted with spurs....something you will need sometime as you progress.