Lie down on a flat bench and with your arms fully extended to your sides hold the dumbbells with your palms facing up. Your arms will be hanging down slightly past your body. This is the starting position.
Raise the dumbbells up until your forearms are completely vertical and then lower the weight back down to the original starting position. Repeat the entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't let the dumbbells pull your arms too far back down; your arms should go down past your body but not too far down
Don't do this exercise if you're a complete beginner
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This exercise is not one that you will see people doing often at the gym. It may feel a bit awkward to execute the first couple of times, but once you get used to you will reap the benefits of this movement. The benefits to your biceps are that you will get your lower bicep head elongated very nicely as this is what this exercise will primarily do.
Also, remember to keep your head fully rested on the bench. Lifting your head up will only put unnecessary strain on your neck muscles. Thus, keep your head down while performing this exercise.