Lie down on a flat bench and position the dumbbells directly above your mid-chest. You may want to have a spotter if your are a beginner or if the dumbbells are heavy.
Lower one dumbbell in a controlled manner until your upper arm is a little past parallel to the floor. After that, return to the starting position and repeat the movement with the other side.
DO
DON'T
Lower the weight in a controlled manner
Perform the exercise in a smooth and fluid motion
Be carefull not to lower the dumbbells too fast because this may cause injury to your shoulders.
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, do this exercise with heavier dumbbells.
This is a more advanced exercise that requires great coordination and balance. It's not the type of exercise that should be performed by beginners.
The one-arm alternation engages and challenges your chest muscles in a different way. This is why this exercise is good when you want to change an already existing chest workout routine and add some fresh exercises to avoid hitting a plateau.
It's also worth pointing out that the heavier the dumbbell(s) the more challenging it gets to keep balance. Thus, you may need a spotter to help you out a bit.
Execute this movement with caution and care. Keep the movement at a steady pace and engage your core (stomach) muscles to help you maintain proper balance at all times.