4 speed ka-booom !

I'm talking about the front rand rear edges of the slider, sorry for the arrow placement, kind of in a hurry when I did it. I taper the back about 30 degrees, but only from the middle of the edge up to the top of the edge, not a whole bunch, but just enough to make a smooth transition. I don't have any pic's of one done at the moment, I have the guy that does my axles harden them for me, I'm not 100% sure which method he uses.

Thanks for the explanation! So you cut the red part in the picture.

This is the first time I hear about this mod. What does it actually do? As far as I know the synchro rings ride against this edge. The slider/sleeve is always in contact with the drum/hub so it wouldn't be effected by the taper - and the synchro rings aren't effected as well..
or am I wrong?

I attached a picture for clarification: