: What is this weird exercise given to me by my Trainer for chest? My trainer told me about this exercise for the chest, but I couldn't find it on BodyBuilding.com What it involve is to have dumbbells on your sides with arm

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Your description could describe a Front Raise

As regards how the Front Raise can target the pectoral muscles, this fellow comments on underhanded Front Raises:

Depending on how you do underhand front DB raises they can be for the anterior deltoid or they can be a focus on the upper peck [sic]. When bringing the palms together at the peak of the movement in front of you its a good contraction on the upper pecks. When going more straight it hits the anterior deltoids more.

Wikipedia says many of the same things in different language (bolding mine):

It primarily works the anterior deltoid, with assistance from the serratus anterior, biceps brachii and clavicular portions of the pectoralis major.

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I highly advise against this.

Biomechanically an underhand grip leads to a chain of events.

Supinating your forearm (palm up)
Results in external rotation of your humerus (arm bone).
Your deltoid (big shoulder muscle) attaches midway down on your humerus.
Your rotator cuff - key shoulder stabilizers, attach to the head (or
top) of your humerus.

The Result:

This alters the resting position of your shoulder stabilizers so they fire late during overhead movements.
Your anterior deltoid (also attached to your arm) is the primary shoulder flexor (when you raise your arm in front of you) an increased shearing force is applied to a now unstable joint / and rotator cuff.
You'll feel this more the higher you raise your arm (the shearing force increases).

There’s actually a lot more going on but no need to go farther.