1) Lie flat on a bench holding a dumbbell overhead in such a way that the space between your thumb and forefinger wraps around the bar of the dumbbell.
	It is strongly recommended that you have a spotter hand you the weight. The spotter should then stand behind your head and follow along with the weight.

2) Slowly begin to lower the dumbbell behind your head by bending backward at the shoulders. Also begin to slowly bend at the elbows as well.

3) Continue until the dumbbell is just at the level of your head. At this point, your elbows should be bent approximately 90 degrees. Do not let your shoulders bend significantly beyond this point.

4) Explode upward in the reverse direction until the dumbbell is hovering over your head as in the first picture. Your spotter should follow along with the path of the dumbbell.

First, be sure to protect your lower back. Do not arch your back as you lower the weight. If needed, prop your legs up on the bench to prevent this from happening. You will find that you probably need something/someone to hold your feet down if you are using heavy weight.

Second, try not to lower your arms or drop the weight backward too far. Doing so can cause some shoulder impingement problems.

You will definitely use your lats to bring the weight up over your head. Look at the motion happening at the shoulder. This is a very similar motion to a lat pull-down. This essentially makes the pullover more of a back exercise.

You will also use your pecs to help out to pull the arm back up. Your triceps will be worked as well as the elbows extend. You can turn this into a more dominating triceps exercise simply be bending/flexing the elbows more.

Then there is the notion that pullovers "expand the ribcage". The only possible way for this to occur would be if the stress of being stretched under tension when the weight is overhead caused stretching of the costal cartilage. This cartilage binds the ribs to the sternum or breast bone. This would essentially loosen up the attachment of the ribs and cause your ribcage to be larger. Otherwise, it is impossible to make your ribs larger. You are much more likely to stretch out your rotator cuff and shoulder joint which can invite numerous shoulder problems down the road.