The fins are like reversible propeller blades; these nuts allow you to adjust the twist rate (on the ST and Turbofins). For best performance, nuts should be adjusted so the fins are "loose", allowing them to twist easily and wind up the mast without restriction. They should tuck in to the "clew outhaul" slightly (about 1/8") and look something like this:
As the fins are drawn up tighter, they become stiffer, pedaling effort increases and more "thrust" goes out the sides rather than the rear. It becomes more like batting boards back and forth. If your fin instructions from Hobie say something about tightening the nuts for higher speeds, this should be ignored.
If using the older style Drive, look for at least 1/4" free movement space for the fins toslide up and down the mast.