The plastic bit clamps onto the end of the strap and it holds the end of the strap rigid by a couple of pins and the screw through the middle.
You screw the bolt into the board and friction and the little spikes on the bottom hold it tight to the grip.
The end of the strap can move by a few degrees but the whole end can't twist around. It's not necessary if you have two bolts to hold the end of your strap down.
http://www.sailboardalley.com/Product-DaKine-Twist-Control-System_52.aspx