http://ddowiki.com/page/Nature%27s_Warrior_enhancements
Nature's Warrior PrE offers four different Beast Stances: Bestial Nature, Fatal Harrier, Reaving Roar, and Four Legs Good. Note that despite calling them "Beast Stances," only Four Legs Good requires you to be in an animal form (or using Avatar of Nature from Primal Avatar) to function; the other three stances work fine in any form. You can take as many as you like, but only one can be active at a time. They stack with SD defensive stance, Swashbuckling (but note SB doesn't work in animal forms), and the combat stances (Power Atk / Precision / Combat Expertise / Resilience). Oh, and IIRC barb Rage disables beast stances, so be careful on any druid / barbs you make.
EDIT: and yes, they are a toggle; map them to your toolbar once you take them.