Stand facing a wall, around one large stride away.
Place your hands on the wall around the same height as your shoulders but slightly wider.
Lean against the wall, keeping a straight line from your head to your feet.
Bend your elbows, bringing your chest and hips in towards the wall and pushing your elbows outwards.
Make sure your body stays straight.
Push through your hands to straighten your arms back out again.
Continue this movement.
Note: the further away your feet are from the wall, the harder the exercise will be.