(1)Depending on your body, duck stance can help or hurt your knees. Some people find that duct puts too much stress on their knees. Others find duct is more anotomically correct way to stand. In theory, the latter is true. Try if your self. Stand with your feet pointing forward (i.e. a 0 stance), and bend down as deep as you can. Next, stand a but duct, and try to bend down as deep you as can. You'll see that you can go MUCH lower when standing like this. Hence, when riding duct, you are able to been through your knees in a much more natural manner. How does this help when riding? Well, I hope that answer is obvious enough. Also note that this has NOTHING to do with switch riding...that benefit is just ancillary.
(2) As already stated, do NOT go straight to 15/-15. It's kind of like lifting weights. You can start out with a 250lbs barbell. Start with a 15/-6, then goto -9, -12, etc. You'll need to get your muscles used to the new stance w/o freaking your body out too much.
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