it's been a while, but i used to have a really nice shifting motor setup
very light first gear shoes and high spring tension for high engagement and light shoes for smooth, slippery engagement
stock second gear because i wanted it to engage 100% and not slip. the weight is what gets you solid positive engagement. then you can set the speed it shifts at pretty accurately with spring tension
Series of events that took place:
first gear shoes are fixed directly to the crank
first gear spins up and slips/engages on the bell
first gear engages the bell 100% and revs build
the motion of the bell spins up the second gear clutch and it engages the bell
the decrease in rpm breaks the first gear shoes free from the bell and first gear slips while second gear is 100% engaged. this allows for a feathered clutch feel in second gear that keeps you in the powerband.
I'm sure you could achieve the same feel with 2 special clutches, but you can make it happen with only one
this is all based on sound, feel, and lots of experimentation but there's definitely the possibility i have it all wrong.
as i understand it, (and the basis for the above) when you're rapped out in second gear, power is being trasfered from:
crank -> 1st shoes -> bell -> second shoes -> reduction