Actually leaning back and away from the lat bar, during the pull down motion, brings into action more lat response. Works extremely well when doing pull-up/chins and is akin to the Gironda method of chinning. Where the higher you rose up the more you tried to distance yourself from the bar. Great lat pump this way.
Chins behind the neck, actually touching the nap of the neck, is also great for the rear delts, upper back/lats. Needs a getting use to, but worth the time, when flexibility is gained. Of course add weight after a while. Good Luck.