Friction locked pins provide the lowest retention in amalgam restorations because they rely solely on mechanical friction between the pin and dentin without additional threading or cementation. This minimal engagement offers less resistance to dislodgement compared to self-threaded or cemented pins, which create a stronger mechanical or adhesive bond. Interference fit pins have a tight fit but still offer more retention than friction locked pins due to the elastic deformation of dentin. Therefore, friction locked pins are less effective for securely retaining amalgam in extensive restorations.

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