if you're saling downwind and you let the rope go it is only going to make you go faster in any kind of sailing. it depends on what is your angle with the wind
No it won’t, if you are running downwind in a Blokart and let the rope go the sail will turn completely with the wind and depower. Even running downwind there has to be tension on the main sheet through the pulley whip for the sail to be powered up.