All of the sounds are produced by the strings on the instrument and how the bow is pulled across the instrument.

所有声音都是靠乐器上的弦产生的,琴弓在乐器上来回滑动发声

耶鲁公开课 - 聆听音乐课程节选

You can do it on other instruments but on a string instrument it's the difference.

很多乐器都能奏出颤音,但弦乐器上　这种揉弦产生的颤音是不一样的

耶鲁公开课 - 聆听音乐课程节选