I made this term up so don't try to find it anywhere else. A "brainstopper" is something you say or do that causes the mind of an audience member to stop to think. This can be a good thing, but most of the time when I catch a coaching student delivering a brainstopper, it is a bad thing.
Here is an example of a good brainstopper. You might say, "Take a moment and think about the first toy you had as a child."
A command like this would take the audience member's mind from where it is now to a time long ago. For most of the audience this will be a pleasant experience. For some it may be unpleasant. Either way you still are directing the show. You might do this to make some kind of point about how simple things used to please us, or something like that.
Here is an example of a bad brainstopper. You might say, "That man's elocution is impeccable." For all of us highly educated and brilliant folks the word "elocution" obviously means fine form in speaking or reading.
If this word was used in a less educated arena, the instant it came out of your mouth, the brains of the audience members would be racing to figure out what the word "elocution" means. Thus, their brains have stopped because you used a word that was not easily understood. The audience member will not hear your next few sentences because they are still trying to figure out the word "elocution." Do this several times and they will tune out altogether ... unless of course you are Deepak Chopra who gets high praise for being totally unintelligible hahahahaha
Another way to stop someone's brain is to distract them by your actions. You might display an odd prop before explaining what it is. This would make an audience member stop listening while their minds tried to figure out what the prop is. If you were talking during this time, they wouldn't hear a word you said.
Look at your word choice and actions carefully before you exhibit them on stage. It is hard enough to keep attention in today's short attention span environments. Don't make it worse by using bad brainstoppers.