The symbol in the middle of the pendant is called the Tibetan knot (Srivatsa or the endless knot) and is one of the eight symbols of the Tibetan Buddhism. The Tibetan knot can stand for karmic consequences: pull here, something happens over there. It is an apt symbol for the Vajrayana methods: Often when we tug at one part of a knot while trying to loosen it, another part gets tighter. You have to work with the knot to get it to come undone. In its endless configuration, it evokes the cyclic nature of rebirth and also calls karmic connections to mind.
The Tibetan knot pendant helps the wearer to realize the natural rule of cause and effect. It also reminds the wearer the need to let go and work with the knot and not against it.