Dms works together with bruises, when you take damage in your joints the more red it gets the higher the chance it has to dismember, every joint dismembers in a specific way, the knee for example almost everytime can dm or frac if you get the right grab.
Fractures most of the time are involved with bad grabs, if your opponent has a grab in one of your joints and you force that grab by contracting or extending it in the wrong moment, the chance of that joint fracturing goes up, same goes for dms.
This is how they work in a very basic way, the dms work the same in basically every mod, the only thing that changes in most of them is the treshold or joint power.