This works the upper part of your chest (upper pectorlis major)
Without changing the angle of the dumbbells, lower them until they are level with your chest in line with your nipples) and without pausing push them straight back up again, keeping your shoulders and head on the bench at all times. At the top connect your brain with your chest and squeeze for an extra second or two before lowering and repeating.
Ensure you don’t lift your hips of glutes off the bench. Some people find they perform step 2 better when they imagine pushing the body down into the bench away from the bar rather than moving the bar away from the chest.