Here is a copy of the instructions I found on the net. I could see simply lacing it through the tow hook and then laying the flat part of the strap through the loop. Place the bone in and as the strap tightens up it will be held in place but **** you could do that with a large wrench, tire iron, piece of round bar, ect. It seems the slit in the bone comes into play but I can not find pictures to support that.
Anyway here are the instructions I found.
Once a tow strap is laced through the tow hook, simply thread the strap through the middle slot of the cinch bone then situate the cinch bone between the end loop and flat portion of the strap to create a firm connection and tie a knot. This keeps the strap from slipping. After use, untie and slip off. No more impossible knots or cutting pieces off of your straps! It is recommended to use two cinch bones, one for each end of strap for reinforcement.
Chris