Lie down on a declined bench with dumbbells placed directly above your chest. If the dumbbells are heavy, you may need a spotter to help you.
Lower one dumbbell in a controlled manner down until your upper arms are parallel to the floor. Then, return to the start position and repeat movement with other arm.
DO
DON'T
Lower the weight in a controlled manner
Perform the exercise in a smooth and fluid motion
Don't lower the dumbbell(s) too fast as this may result in injury
Don't jerk the dumbbells on the way up
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
If you want to increase the level of difficulty, simply select heavier dumbbells. However, when the dumbbells get too heavy you may need a spotter to help you get them into position.
This is a great exercise for the inner chest area. Keep in mind that this is not a muscle mass building exercise - it's a movement that will give you better definition and muscle separation in the inner chest area. This is the exercise that will give you that "line" down the middle of your pecs and give your chest the appearance of having two separate superhero-type chest "plates".
Thus, focus more on form and pace and don't worry too much about going heavy on this exercise. It's a finesse movement and should be treated as such.