Can anybody explain the theories behind back pressure?

I have the understanding that back pressure means absolutely nothing to engine performance.. I think of it as a misused term for slowing down the exhaust gasses so that the pulses can line up enough for scavenging to come into effect (leading end of pulse is very high pressure and trailing edge very low pressure.. if the trailing edge is close enough to the leading edge of the next pulse then it will cause a pulling effect). If there is unrestricted flow does that mean that at low RPM's the pulses are too far apart from each other for scavenging to come into play.. while at high RPM's they are? If true, then it would mean that restricting flow would keep the pulses closer together at low RPM's but making it much harder on the engine to expel the exhaust at higher RPM's right?

 

i have no clue what you are talking about pulses. but the way i was told about pressure is that if you have to much it hurts your mid to top end power and if you dont have enough then you will have no low end torque. thats what i was told

 

What's the old saying? Big blocks use small header small blocks use big headers?