What exactly does a shorter back fin (speaking about trifin raceboards) bring?
More loose back feet? Better upwind? More (?) control downwind?
And how much shorter should (if) it be, 2-4cm shorter than the frontfins?

Thanks for your input

if the frontfins are bigger they keep your nose down
the board gets less nervous and bites more for upwind

So how does that work then??

basically a shorter backfin keeps the board in line and have less drag means the board is faster with a short back fin but the main work is done by the longer trusterfins

how long they have to be in comparison to the backfin u have to try that out
there are a lot of influences like wind strength/ waterconditions/ kitesize which kind of kite u are using/ board length and of course your technique and your weight