My two cents. To try to keep it simple. If the boom is sheeted to the centre of the boat, ie no traveller, all the force is straight up and down, which flattens the sail. The further you let it out the greater the force in the horizontal direction and the less vertically, so the sail becomes less flat. Draw the triangle where the hypotenuse is the mainsheet.
Put in the traveller and as the boom goes outboard to leeward you can keep the pull vertical, to a certain point, because the traveller isn't wide enough to cover the full range.
Without a traveller as you pull the boom in you increasingly pull more vertically and get a flatter sail, and as you let it out you get less vertical pull and a fuller sail which you may not want.
The practical effect of this is that you set the mainsheet and traveller to get the sail shape you want and then leave the mainsheet and trim via the traveller.
The shape in light winds is a balance between twist via less vertical downpull, and traveller to windward, and flatter shape in the very light, then more shape as it gets a bit stronger set through other controls ie outhaul, cunningham and halyard, through to flat shape with stronger winds using all those plus the traveller to leeward.
When I put a traveller on a class trailer sailer the effect was dramatic, in pointing ability, speed and standing up. You could see the results in racing. There's nothing like carving through the fleet as feedback.
In essence then use the mainsheet and the other controls to set your shape for the wind and conditions and your traveller for trim. This is the opposite to just using the mainsheet, with the traveller as only semi-moveable. It is also useful cruising, though then you don't have to trim so often.
I hope this attempt to explain the basic principles helps though after three years the original poster probably has it sorted lol.
Last edited by chris_gee; 02-15-2007 at 10:42 PM.