saw off the bough on which one is sitting

例句：

1. 1. He saw off the branch with his axe.
   他用斧头砍掉了这根树枝。
2. 2. The car was saw off by the police.
   那辆车被警察截停了。
3. 3. She saw off the rope and freed herself from the noose.
   她用绳子割断了自己脖子上的绳索，从而获救了。