Shadow technology is already present on many of Shimano mid-level and high-end derailleurs. Basically, it’s all about the shifter’s low profile, with the cage working inside the plane represented by the chain stay and dropouts. This really helps on the track, as with the extra clearance, chances of clenching it to various obstacles are lowered.
However, the Shadow Plus RD is something really different! Shimano explains it to us in detail: it features a switch at pulley cage allowing you to enforce a heavier spring tension. The system works the same way as a clutch, facilitating a very accurate positioning of the cage. This also means that chances of dropping the chain are minimum if not quite zero.
Also, the folks from Shimano show us that with the RD Plus system activated, it is difficult even to move the pulley cage by hand. A small disadvantage comes when moving up the cassette, as shifting becomes a bit heavier, as the lever becomes slightly heavier to operate.
Another advantage is that you can adjust the tension of the pulley, by removing the plastic cover, illustrated in green in the image below. Inside the derailleur one can find a small key to adjust the tension (basically by tightening the friction band connected to the P-knuckle). The tenser the cage, the chain slap while riding on rough trails is significantly reduced.
Shimano took even further the concept of silence functioning and installed a bump stopper (between the RD and the dropout) on the new Saint derailleur in order to reduce impact noise.
See Shimano XTR Shadow Plus derailleur in action in the movie below: