Sit down on a preacher bench and rest your elbows and upper arms comfortably on the pad. Grab a hold of the dumbbells and hold them up "hammer" style. This is your starting position.
Lower the dumbbells down until your arms are almost fully extended (but not completely because you always need to keep tension in your biceps). Then, raise them back up until your forearms are almost completely vertical. Repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely on the downward portion of the movement
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is a spanker of an exercise. Thus, it's certainly not for beginners. It's a great movement for building that lower head on your biceps that many people often neglect. The elongation that you will get on your biceps from this movement will give you a nicer and fuller look to your arms.
The trick with this exercise is to go heavy with the dumbbells because you need to challenge your biceps as much as possible. Doing this exercise with very light dumbbells won't do much for your arms other than tone them. But, that's not the main purpose of this exercise. This exercise is a spanker because with heavy enough dumbbells it will shock and awe your biceps into further growth. And that's what (most of) us want!