The Thoracic Rotation exercise is performed on your hands and knees (in other words, in the quadruped position!). This movement focuses on requires stretching and mobilising the upper and middle back. It is a good exercise to relieve back stiffness and tightness before you start training.
Start on all fours with your hands and knees about shoulder width apart, and sit back towards your heels to take the pressure of the lower back. Place on hand behind your head and bring your elbow to your opposite hand, then rotate back up so you feel a stretch in the upper back. Keep the range of motion comfortable, no need to push too far. Repeat on the same side before switching.