Step outside and you could break a leg slipping on your doormat.请问其中的slipping是什么作用？

1. slipping on your doormat为现在分词短语做条件状语。

2. 即：Step outside and you could break a leg slipping on your doormat.

= Step outside and you could break a leg (if you slipped) on your doormat.

此条件状语从句采用了虚拟语气，只是一种假设的情况。

3. 因为非谓语动词slip和句子主语you之间为主动关系，所以用现在分词slipping.