HOW: Place a step in front of a wall or an object to use for balance. Using only one leg, step onto the box with the balls of your feet on the edge of the box, and let your heel hang off the box. While keeping your glutes and quads tight to keep your knee straight, lift your heel up in a slow and controlled manner, then slowly lower your heel back down to the ground. Repeat
FEEL: You should feel your calf getting a workout, but also your glutes and quads working as well to help keep your leg straight.
COMPENSATION: Do not let your knee bend as that is a different exercise. Make sure you go through your full range of motion at your ankle. This is not a balance exercise, so hold onto something to help with balance as the goal of this exercise is to strengthen your calf!