I know enough about engines... the deal is.. you want to continually vary the rpm (not too high) and make sure the engine is completely warmed up first..

its because of the cylindar rings and the cylindar walls need to be worne in

I think car engines are broken in at the factory so its less important.. but smaller things (motorcycles, ect.) need it.

you need to seat your pistons so there is no blow by/loss of oil ect.



also, it is better to break it in when its cold outside, less chance of cylinda glazing (oil cooked onto sides of cylinder), less chance of overheating(because of increased friction), and also less chance of rotor warping (brake rotor)