The primary retention mechanism for friction-locked pins is the resiliency of dentin, which allows it to deform slightly and grip the pin securely. Unlike rigid enamel, dentin exhibits elastic properties that enable a tight frictional engagement when the pin is inserted. This microscopic compression of dentin around the pin creates retention without relying solely on mechanical interlocking or cement adhesion. The undercuts in pin design aid in retention but are secondary to the dentin’s ability to flex and hold the pin firmly in place, preventing dislodgement under functional stresses.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.