[Zeke]

Does the same thing an expansion chamber does on a 2-cycle motorcycle or kart engine. Dave's explanation is good. There is a bit more. Even head pipes that are not merged have a effect on the performance. Too short or beyond the optimum length and performance will suffer. So, if the exhaust has to go any further, it is "collected" in a larger secondary pipe and run to an exit point. So, these "pulses" are staged in the primary pipe and them again in a secondary pipe to keep the gases from building uneven and out of time pressure which unbalances the system. That can lead to some cylinders to run lean and others to run a little richer. The design of exhaust systems for the real environment, i.e., within the body work of a Formula 1 car, is high art. That, as opposed to the 8 big single pipes on a fuuny car. Even those have a "tuned" length.