What do we call the passage of music in which the beat slows down?

对于曲子中节拍慢下来的部分,我们怎么来描述它

耶鲁公开课 - 聆听音乐课程节选

I want to do this carefully and slowly, and before I even start, let me point out in passing that calculus is not going to help us here.

我要慢点仔细讲这部分,在我开始之前,我需要说明微积分,在这里是用不着的

耶鲁公开课 - 博弈论课程节选