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". This is your starting position.
Lower the dumbbells down until your arms are almost fully extended (they should be slightly bent at the bottom of the movement) and 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 when you lower the dumbbells down
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is a very effective exercise for the biceps. However, it's not for absolute beginners because the movement is not as simple to execute as it looks. The synchronicity of the two dumbbells is essential to the effectiveness of the exercise. Thus, if you do happen to be a beginner and still insist on doing this exercise, start with a lightweight pair of dumbbells for a few weeks until you get used to the movement and the balancing aspect involved in doing this exercise.
If you're a seasoned and experienced lifter, this exercise will be the perfect plateau-busting mixer to throw into an existing bicep workout routine!