The following text and photos are courtesy of Al. Most sailboats are designed so that the helm is balanced in light air, but weatherhelm increases as the angle of heel increases due to the force of the wind on the sails. This causes most boats to round up into the wind so the rudder is used to keep the boat on course. The same force on the sails that cause this rounding up, can be transmitted directly through a line and blocks to the tiller, to keep the boat on course. The design of the boat, the choice of sails, the range of wind velocity, or the desired course, are all factors in determining which setup of the lines and blocks or the elastic is most likely to work.
Success is therefore dependent on experimentation for a given boat.
By observing how it works or doesn't work with a given setup and leverage or adjustment of elastic, one gains an intuitive sense that helps reduce the number of adjustments required to get it right. I've always been fascinated by physics and I find the challenge of being able to self steer an enjoyable extension of sailing. Others, I expect are exasperated by this same aspect. But, for a coastal cruiser who already has an autopilot, or a cruiser with a limited budget, or even one who has a vane but would like to have a backup steering system that won't drain the batteries, this could be the answer. Six snap hooks.