This is a Martingale Collar (or Premier)….. it's a no-slip collar! It tightens if the dog tries to slip her head out of the collar while you have her on a lead. I use it for all shy or feral dogs. Often, I use two martingale collars! If I drop one leash, I've got a second one attached to a collar, so the dog doesn't escape. Always have a PLAN A, B and C for the containment of shy dogs!