A couple of months ago I wrote about steering with Sheet-to-Tiller Self-Steering. At the time, I didn't have a good picture of how the arrangement worked. Here is one, showing the aft part of Issuma's cockpit, looking towards the port (windward) side.

In the picture, the control line is tied to the main sheet. The control line leads thru two blocks (only one block is necessary) to the tiller. Strain from the pressure of the wind on the mainsail results in a pull on the control line which attempts to pull the tiller to windward--turning the boat away from the wind.

The bungee (elastic) in the lower left of the picture goes from the tiller to the leeward side of the boat (attachment not shown). The elasticity of the bungee tries to pull the tiller to leeward--turning the boat towards the wind.

We have steered with sheet-to-tiller (from various sails) for over 2,000 miles so far on this trip. When the wind is stronger, we use the windvane (it has many advantages, but does not work well in light wind). When the wind is light, we use sheet-to-tiller.