it lowers the velocity of the air at the emulsion tube at part throttle so you don't get as much vacuum.
when the slide is all the way up, the cutaway no longer matters because its out of the way and the main jet fuel delivery is the same
its most effective from like 0 throttle to about 1/3 throttle on an SHA but it still has some effect all the way up to about 3/4 throttle
its a very very useful mod, if you read the mikuni handbook that explains it pretty well, but obviously it behaves differently on a single jet SHA than a multi-jet VM
once you see how big of a difference it makes you'll cut the slide more or less in every SHA bike you own