backpressure is bad all it does is restrict flow which ultimately reduces power.
The proper term is scavenging
The idea is to have the exhaust gasses moving as fast as possible to make a low pressure in the pipe which helps to more efficently "suck" the gasses from the cylinders.
There is a trade off, a small pipe will yeild good power at a low RPM but will be restrictive at high RPM, and a large pipe will do the opposite.
Another aspect that is often overlooked in designing an exhaust system is that the noise actually helps move the gasses. The sound waves moving through the pipe actually help to keep the gasses moving. Why do you think race cars are so damn loud?
