Set-up: Begin on all fours, and pike your hips so your torso is relatively straight.
Movement: Place your feet relatively close to your body to start. Position one foot flat on the ground, with the second foot gently resting on top of the planted foot's ankle. Gently rock forward and backward so your heel touches the floor and then raises off of it. Slowly walk your feet backwards if you feel comfortable with the stretch and your mobility improves. Be sure to repeat on the opposite side, though you may perform more sets on the tighter leg if you have one.
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.
Instructions and effects for the Rocking Ankle Mobilizations exercise 
Movement in Sagittal Plane