an over-center traveler setting on a monoslug is also technique to depower and increase the stability of the boat. Proper trim at high winds dictates a lot of twist in the main, over centering the traveler allows you to keep the twist needed while setting the sail to deal with the wind. The twist is needed to accommodate for the bobbing and weaving in waves/chop created by the wind. I learned and experienced this while studying up to sail an International Tempest and got to test it in stoopid wind until "Sippy Boy" knocked the compass out of the deck with his butt and the boat started taking on water in every knock down.

We need this twist less because our apparent wind is significantly higher - the bobbing and weaving in the waves has less negative effect on this higher apparent windspeed.