This movement may be performed with a smith machine or a barbell positioned on the bottom of a power rack. Set yourself up for a regular push-up (body straight, abs tight, etc) and grab onto the barbell. Perform a push-up. To increase the difficulty, you can elevate your feet or make the movement weighted (vests, chains, etc). You can also make the movement easier and strengthen your push-ups by putting the bar higher on the rack (upper body higher than lower body).