HOW: Grab a dumbbell of the appropriate weight an assume a supported leaned over position on a bench. The key is that your chest is completely parallel to the bench. Bring your elbow to your side and while maintaining that position, slowly lift the dumbbell behind you by straightening your elbow.
FEEL: You should feel your tricep muscles working.
COMPENSATION: You do not have to use a very heavy weight with this exercise if it is done correctly. Make sure your torso is parallel to the bench.