如果when从句的主语和主句的主语是同一个（些）人，而且从句的谓语动词是“be / am / is / are / was / were + doing ”，可以把从句的主语和 be / am / is / are / was / were 一起省略，使用 when doing sth结构。例如：

Don't forget to signal when turning right.(= ... when you are turning right .) 向右转弯时，不要忘了打信号灯。（摘自《牛津英语用法指南》第三版73.4节）