Thanks Jeff for the explanation! I've started to use the traveller over the last year and to adjust the jib sheet lead to try and match wind twist. However the idea of over twisting the jib to spill wind high up and avoid over powering (assuming I understood correctly) was a useful addition for me....
I was also interested in the need to over-tighten leaches with sails that have UV strips. I've noticed this on some of the boats I rent and was wondering if perhaps I was missing some other way to tension the leach without developing that ugly "hook" in the sail. Is there a way around this problem?