Pectoralis Major, Pectoralis Minor
Check out anatomical considerations here: Pectoral Muscles
Placement: On your Pec. Go ahead, put it there.
Pectoral Release Level 1: The Biker Signal
Exact placement varies depending on pec size, and how much weight you bench. If your pec is sore, this will be a lot of pressure. For an active massage, move the hand slowly back and forth like a biker signaling a turn.
Pectoral Release Level 2: The Crucifix
This one is called the crucifix because the pec may die here. It is a lot of pressure, but great to tear up a tight pec.
How to know you've released a trigger point: When you release a knot/trigger point, you will feel the muscle give and will simultaneously feel the device sink in to your body. A massage generally feels less painful under higher pressure once a trigger point is released.
 Donnelly, Joseph M. Travell, Simons & Simons Myofascial Pain and Dysfunction: the Trigger Point Manual. 3rd ed., Wolters Kluwer Health, 2019.
 Davies, Clair, and Amber Davies. The Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief. 3rd ed., New Harbinger Publications, Inc., 2013.