You reach out and put your thumb on the back of the biner and pinch, forcing the rope (trapped between the gate and first finger) into the biner.

• 这在斜板地形较不顺手，因为快扣靠在岩壁，绳子会被岩壁妨碍到；不过，好处是当你有点要滑动时可以"抓住"快扣，且顺势带出贴紧岩壁的绳子。