How does stretto build intensity in a fugue?

In the context of a fugue, stretto builds intensity by overlapping the subject with its own answer before the first statement has concluded.1

By initiating the subject in one voice and having it proceed immediately in another, the composer creates a dense, layered texture. This technique is frequently employed near the end of a fugue as a deliberate device to drive the music toward a climax. Because the voices enter in close succession, the musical tension increases, heightening the listener's sense of anticipation and momentum.1