I asked a question about this same concept recently but this question involves the function of the bypass air.
In this YouTube video around 1 minute in the narrator says that the bypass air is pushed by the fan into a smaller and smaller space which increases the velocity of it, but why? Why increase the exit velocity of the air beyond what the fan already does? It can't increase thrust, can it? That would seem to violate some sort of conservation law since you could theorize an infinitely narrowing space and thrust would approach infinity. So what function does it actually serve to force air into an 'ever narrowing space'?