Slobber straps go on the rings of a bit usually a snaffle and mecate reins. They are used for pressure release training because of the added weight. This is from www.naturalhorsesupply.com
Slobber straps are a double strap of leather with a hole in both ends. They are normally used with a snaffle and mecate reins. The slobber straps are folded and fit through the rings on the bit. They are designed to be used with a bit that has at least 3" rings so that there is room for the headstall and slobber straps.
The rein is attached through both straps with a half hitch knot which allows for easy adjustment and removal -- no matter how hard the rein gets pulled, this knot will easily loosen.
Slobber straps are especially useful to avoid wear on your mecate reins and they keep the reins out of the water when your horse gets a drink. The weight of the slobber straps assist with the mecate reins in providing an instant release (reward) to the horse when training. This is essential when you consider that you only have 2-3 seconds to reward your horse when you get appropriate behavior before the horse looses the concept of relating the behavior to the reward.