This exercise is a variation of the bench press, which focuses on strengthening the triceps.
Begin by laying down on a flat bench press. Next, reach up and grab the bar with a grip approximately at shoulder width. Your grip should be with your palms facing your head; this is the opposite of a normal bench press. Remove the bar from the rack by pushing up and bring the bar out and over your head. Now, slowly lower the bar to your lower chest and make sure to keep your elbows in tight to your body. Pause slightly at with the bar on your chest and then push the bar back up to the starting position. Focus on using your triceps to push the bar up and lock out your arms at the top position. Repeat this movement until you are done with your set. Then, carefully return the bar to the rack.
A spotter is very useful for this exercise; it is difficult to get the bar off of the rack with a reverse grip, so a spotter could help you. This exercise is more difficult than a standard bench press, so you will have to use less weight.