InsideTech

To rein means to pull on the reins, to slow down a horse. Often accompanied by the command "Whoa, Nellie!" So I take it you are asking which OS is retarding forward progress. LOL

Either that, or "Reining is a western riding competition for horses where the riders guide the horses through a precise pattern of circles, spins, and stops." Which is hard to parse in terms of operating systems, so I doubt that's the sense you intended.

The reigning system, on the other hand, would be the dominant one.