you're letting the exhaust gasses flow quicker through the system, allowing the engine to 'breath' better..
but if you've got a little ciblet 250, just slapping on an aftermarket exhaust can won't do anything except make your ears bleed. Ideally you'd want to compliment the freer flowing exhaust gasses by having a freer flowing induction ie...a unifilter that allows more air through the carbs. however, allowing more air through without sufficient petrol will make your engine run lean and a lot hotter ( you don't want that) so you need to compensate by introducing more petrol into the air/fuel mix and you achieve this by installing bigger 'jets' in the carburetor. *******?