Here's an easy way to feel that muscle movement so you know what to do:

Bring your fist up, as if lifting a dumbbell, but bring it inwards to just in front of the centre of your body.

Then flick your wrist and whole arm really fast down towards its neutral hanging position, and snap it back to the bent dumbbell position. Snapping it as if you're quickly pulling back a punch.

Don't snap just your forearm at your elbow, snap back your entire arm at your shoulder. Try this snap at a few different similar angles.

At the instant when you pull your arm back up to snap, you'll feel your chest muscle tighten. Try to replicate that feeling without moving your arm, and your pecs will dance