Hold onto the dip bars and raise your feet off the platform. Then raise and lower yourself by bending or straightening your elbows. This exercise is often used primarily for the triceps, but also is great for the lower chest. For more concentration on the triceps, use a larger range of motion, almost locking out your elbows on the way up. For more emphasise on the chest, lean forward more so that your pecs are doing more work.