George,

 

My WAG about why the notch on the slide is that the air spilling over a flat edge probably has a lot turbulence, the notch may tend to reduce it.  Also the notch naturally affects the area of the opening as the slide moves across the bore.  It adds more open area early on as the slide opens, this may made the area opening "more" linear than a flat edge opening.  But, as I said just WAGS

 

Ed

Ed,

Seeing everyone's as much in the dark about this as I am, I will tell you what I thought. My opinion is that is adds a little extra area in the lower RPM range especially idle and concentrates the incoming area around more of the needle, thus increasing the signal ( increasing the needles negative pressure) by exposing more needle while limiting total area.

Could also be way off the mark.

George ( down under)