H-Pipe vs. X-Pipe vs. Straight-pipes

The exhaust pulses alternate from the left side to the right side alternately (as the cylinders fire). When you use an X-pipe, it creates a scavenging effect where the pulse from one side basically helps suck the upcoming pulse through the other side. Hope that makes sense, I'm sure someone can explain better. An H pipe has somewhat of the same effect, but not as much.