I am going to input my $.02 on this one. Any guy who has ever been BLUE BALLED knows exactly what back pressure is!
On a more serious note. Back pressure is created when the volume of exhaust gas leaving the combustion chambers,exceeds the capability of the exhaust system to release exhaust gas. This lack of ability to release exhaust gas can take place at any point in the exhaust system (normally cat.). If the exhaust system is not capable of releasing that volume of air before the next exhaust valve opening, then the exhaust system will continue to bulid "back pressure". Most production exhaust systems will maintain around 2 lbs to 3 lbs of pressure on acceleration. Anything over about 4 lbs is excessive. If there is excessive restriction in the exhaust system then the "back pressure" will continue to bulid until it equalizes with the pressure in the cylinder putting back pressure against the piston. When the intake valve opens this pressure will release against the volume of fresh air trying to be drawn (normally aspirated) or forced (super charger, turbo) into the cylinder, reducing the engines ability to pull fresh air into the cylinder and affecting engine vaccum. This is why a restricted exhaust can cause a mass air flow sensor code to set. The computer sees throttle angle and engine load but not enough air being drawn into the engine so it thinks the maf is not working properly.
Scavenging takes place when an exhaust system is tuned to create a negative pressure back pulse in the exhaust system. this negetive pressure creates a vaccum to draw the inert gas from the combustion chamber,while at the same time drawing fresh air in through the intake valve on overlap for a few deg. before top dead center. I believe that a very slight exhaust back pressure in the exhaust system (tuned exhaust) between exhaust valve openings creates a vaccum in the exhaust sytem which helps draw exhaust out of the combustion chamber. As the exhaust pressure is leaving the exhaust system it creates a venturi vaccum effect which draws exhaust gas from the combustion chamber.
Thats my $.02.