The source of the segol was the Alcalay big dictionary from 1965. I just checked it again because I was wondering if you put a different organ than "heart" after תשומת, would you get another expression? Sure enough, he has it with יד hand, and it means loan, pledge, deposit, or guarantee.
I agree with airelibre here. This is something that teachers say about lectures, but I imagine stand-up comedians, priests giving sermons, and others who are trying to hold the attention of a crowd probably use it to. I might look at a written lecture and point out "here is where you lost their attention."