He is at his best when he speaks off the cuff.
他即兴讲话讲得很出色。
off the cuff: without preparing ahead of time what one will say; extemporaneous. 无准备地,即兴地,即席的,不用讲稿的
再如:
Considering that the speech was off the cuff,it was rather good.
由于这发言是毫无准备的,所以它还算是不错的。