Dead bug

Lie on your back with arms reaching up and knees bent over the hips, then slowly lower the opposite arm and leg toward the floor while pressing your lower back flat. It teaches the trunk to stay still while the limbs move independently, which is what controlling a turn asks of it: legs working underneath, upper body quiet. The photo shows a simpler legs-only drill from a similar position, with the arms resting on the floor; add the arm reach once your back stays flat. It is gentle on the back, so it suits the first week of the plan and makes a good warm-up for the harder strength work later.