The M981 is the no frills and lightest XTR rear derailleur that Shimano produces. It is basically the same as the Shadow Plus without the Plus. This means there is no clutch mechanism which dampens the movement in the cage when traveling over rough terrain. If lightweight and minimal design is what your after in a high-end Shimano mtb rear changer, this is a great choice.
The M981 features Shimano's Shadow design where the derailleur is tucked in under and behind the cassette where it is less likely to be damaged from the perils of off-road riding. The shifting performance is also improved due to the closer position to the cassette. The Shadow design places priority on a low profile with less body movement for increased performance and durability. When in high gear, the derailleur has a profile that is a full 15mm less than a traditional rear derailleur. That means it barely protrudes more than the quick release skewer nut.