It has the effect of reducing the slide cutaway and enriching the off-idle transition. The circuit it effects is between the idle circuit and the needle taper/position. It should be most pronounced at about 1/8th throttle. (Just like going from a 3 1/2 to a 3 slide on an Amal.)
If you have a free flowing exhaust and intake and you have an off-idle hesitation or flat-spot, then the mod should help improve throttle response.