A deployant clasp (also spelled deployment clasp) is a hinged, folding metal buckle for a watch strap or bracelet. Instead of threading a strap through a pin buckle, you unfold the clasp to take the watch on and off, and it folds shut to a preset size.
How it works
The clasp is built from hinged metal blades that fold flat against your wrist when closed and expand open when you release it. Once you set it to the right length, the strap keeps that size — you just pop the clasp open to slip the watch off and snap it shut to put it back on. Your leather strap never has to bend through a buckle, so it lasts longer.
Why people like them
Three reasons. It's secure — most deployant clasps click firmly shut and won't come loose on their own. It's convenient — on and off in one motion, no fiddling with a pin. And it protects leather straps from the creasing and cracking a traditional buckle causes over time. The trade-off is a bit more bulk under the wrist and a slightly higher price than a simple buckle.
Types you'll see
There are a few styles: a folding clasp that opens on one side, a butterfly (double-fold) clasp that opens from both sides and hides completely when closed, and versions with a safety lock or push buttons. Bracelets usually come with a deployant clasp built in; leather straps often let you add one in place of the buckle.