Pectoral muscles are the 3rd largest muscles of the human body (after legs and back) and the easiest ones one can develop impressive control over.

Pecs are comprised of two muscles: pectoralis major and pectoralis minor.

Pectoralis major is actually the one we see and call "pec" while the pectoralis minor is beneath the pectoralis major and assists in rib cage movement. Apparently, the pectoralis major is the one used in pec control performances. 

Moving in a more detailed approach, pectoralis major consists of two major heads (clavicular and sternocostal or just sternal on the left image) with only one traditionally used in pectoral control skills (sternocostal). There are, however, rare occasions where the clavicular joins the sternocostal in pec control performances.