Quick-release is not a recent invention: Piaget was already using small internal pins in the seventies to facilitate bracelet changes on ultra-thin movements, where every tenth of a millimeter mattered. The concept has remained the same; mass adoption came later.
The mechanical principle is straightforward. The traditional spring bar compresses with a dedicated tool inserted into the lug hole. The quick-release version adds a side pin, a lever, a button, or a bayonet mechanism that allows the same compression from the outside, with your thumb or fingernail, without tools. The elastic force holding the bar in place doesn't change: only the way you overcome it does.
The solutions in circulation are numerous and non-interchangeable. Classic quick-release spring bars feature a tab protruding from the bar's side: you press it, the spring retracts, the strap releases. Manufacturers' proprietary systems work differently: Panerai uses the PAM Click with pushers integrated into the final link; Hublot has a one-click system with a button on the lug back; Audemars Piguet patents pushers on the strap back itself; Rado calls theirs EasyClip. The Straum Coupling, used by some microbrands, is a separate Grade 5 titanium endlink that acts as an interface: you replace the endlink, not the spring bar.
The practical difference between a well-executed system and an approximate one is immediately visible. The bayonets on the Tissot PRX, often cited as a negative example, are technically quick-release but physically inaccessible because they're wedged between caseback and bracelet: the mechanism exists, the ergonomics don't. A good quick-release must be operable with one hand, without placing the watch on the workbench.
From a structural standpoint, the critical issue is resistance to lateral tension. A standard spring bar works primarily in axial compression; the quick-release tab introduces a point of reduced cross-section. On quality steels the compromise is acceptable. On budget implementations, that tab is the first place to look for deformation after a few years of intensive use.
Industry standardization doesn't exist yet. Two watches with identical lug widths can have completely incompatible quick-release systems if manufacturers have chosen different geometries for the internal pin housing.