I've seen a hundred descriptions for this one exercise, so I'll post the one from Dave Tate over at elitefts.com...
– This has to be one of the most misunderstood movements we do. As you will see people are making this too complicated. To perform this movement you lay on the bench the same as you would a bench press. Take a medium grip on the bar and un-rack the weight. Start the bar so it is directly over the upper pecs – neck region. Now, picture a plum line dropping from the center of the bar straight down to the upper pecs. Now lower the bar down that line while keeping the elbows tucked. DO NOT GO OFF THIS LINE! At about the ½ to ¾ point you will stop the bar. At this point a beginner should press the bar back up until they master this. If you have this down then you will let the bar rotate back ½ to 1 inch then press the bar back up. This is one of the best if not the best triceps movements you can do.
This works so well because it strengthens elbow extension strength while imitating shoulder rotation.