Triple buffering
Three image buffers instead of two, so the GPU can start the next frame without waiting for the display's blanking interval.
With double buffering and vsync, a frame finished just after the blank must wait a whole refresh before it can be shown, and the card sits idle meanwhile. A third buffer lets it keep drawing into the free one while the finished frame waits its turn.
The cost is more memory and, in the ordinary swap-chain variant, a frame of extra latency, because a completed image can go stale before it is displayed. Variants such as NVIDIA's Fast Sync and AMD's Enhanced Sync instead discard old frames to keep latency down.