Apparently it works by delaying frame rendering until needed for next refresh, instead of immediately starting work on the next frame when previous one is ready. It's non trivial to do reliably so enabling low input lag might result in stuttering if the window gets missed - CSGO was awful with this with the first low latency mode drivers, but that has since been fixed.
I'd keep it on globally and disable it game per game basis if you run into stuttering.