it'll allow the driven clutch to retard or advance the upshift process slightly,
based on de/increased amount of engine torque needed to push the sheaves apart.
Basically, "best" is to know what you're looking for.. if you want to accelerate more quickly and you run a lightweight kart on a flat road more often than not.. then the first hole is your target...
if you on the other hand have a heavy offroad kart that you want to easily climb a hill,
crawl over sticks and stones and such... then the last hole is the dimple of choice, to give you that little bit of extra torque you'd need.
If you're one day rolling the roads and the next mudding the lawn.. then the center (default) is what you should keep..
All three provide the same top speed and maximum take off torque;
all start out at max torque at ~2200 rpm and provide full speed at ~3600 (or max rpm for that matter)
but if it'S the very same kart and driver...
the first hole let's it shifts up @ 2600 rpm, the center @ 3000 rpm and the last @ 3400 rpm
(numbers are totally made up

it shifts based on load; not speed... but that'll give you an idea)
'sid