[bostik]

It's called "not stepping on a laugh".

When you land a punchline, you sweep the crowd for a few seconds. If you then start a new one before the laugh has finished, you are not just breaking the illusion of having told the joke to them - you are rudely interrupting them enjoying themselves.

Also: while laughing, the audience will miss the next feedline / lead. The joke you started during a laugh will not land well, no matter how good it might be.

I did a 6-week course early this year at the Angel Comedy. In the showcase, the joke of the show came in a young Danish-Chinese woman's set, and she played it perfectly. 25 seconds of laughter, in three rolling waves - and she had the guts to ride it through. All of us were congratulating her afterwards.