“骑虎难下”这个成语用来比喻做事中途遇到很大困难，但形势所迫，又不能中止。
When One Rides a Tiger It is Hard to Dismount
Yang Jian was the regent of the last king of Northern Zhou(557-581).His wife advised him:"Northern Zhou is dying. Now it is as if you are riding on the back of a tiger. It will be dangerous to dismount. You can do nothing but continue." Yang Jian thought this quite reasonable. Later, he founded the Sui Dynasty, and united China once more.
This idom is used as a metaphor meaning that one is in a difficult situation and cannot help but ontinue to pursue one"s sourse.