At these points, the improvising soloist (usually singular) fills the otherwise empty sonic space with dramatic solo obligatti
To get a sense of what Anderson is talking about sonically, check out Louis Armstrong's sublime set of breaks on "Potato Head Blues" (1927) with the Hot Seven. Go listen to like two hours of the classic Hot Fives and Sevens. If you just want to catch the breaks and have other priorities (sigh), here's a link that skips to the breaks.