Set up a bar in a squat rack at about chest height.
Stand in front of the bar and grip it with a shoulder width stance. Step back about three feet (more or less depending on your height), position your feet together and extend your arms. You will be leaning on the bar at this point which will be the starting position.
Begin by bending the elbows and letting yourself lower towards the bar.
Quickly pause and then push yourself back up via the triceps and extending your forearms.
Repeat for desired reps.
You can add chains over your shoulders once your body weight has become too easy.