Adaptive Vsync: if you fall below the target frequency, v-sync is deactivated.
Vsync triple buffered: v-sync at all times.
I ask again, what's the point? Be that as it may, it does not reduce performance. The only difference is that if it were adaptive v-sync in those circumstances (50fps) tearing would appear when v-sync was disabled (although sometimes it is better because it reduces the latency of frame drops). This is easy to solve, try playing any game to activate and deactivate vsync, whether adaptive or triple buffered, and you will see that the performance will be the same.