It's a ghost ring, sometimes seen on the timing chain of a tandem instead of the more common eccentric front BB, also on singlespeed conversions with vertical dropouts where there is no magic gear solution.
Efficiency should be slightly better than using a sprung tensioner with a jockey wheel, as the ghost ring has exactly zero bearing losses and the chain roller and side plate friction would be about the same.
It's a ghost ring, sometimes seen on the timing chain of a tandem instead of the more common eccentric front BB, also on singlespeed conversions with vertical dropouts where there is no magic gear solution.