Hi Silkreeler37,
Are you saying that you trained at a YMAA school and learned brush knee that way?
I might be misinterpreting what you're calling 'twist the body' and what you're saying about yin-yang hand posture, but in the YMAA version of brush knee, the body absolutely does twist from an angle to a squared-forward position as the weight shifts forward, and the palms also do turn to face each other before the step as part of a coiling motion (as the body is twisting).
I don't have time to search around too much- here's the first video of a YMAA student doing the form that I could find. Sorry it's quick! At around 25 seconds he does brush knee, pretty much the way all YMAA folks I've known do it. Is it missing the things you're talking about?
http://www.youtube.com/watch?v=ynU6pazhgzUI think the palm forward/down thing you're noticing has to do with Dr. Yang's interpretation of An energy, 按, which emphasizes settling (in this case of the wrist) as I understand it. The finger tips lead out, and the wrist settles at the end. In my experience, this also means that your hand is less tense during the move, but that's totally subjective for me.
I hope that is helpful or at least relevant!