What a fantastic video of Master Mah! I haven't had a chace yet to look at the rest of these videos, but I'm certainly going to.
Thanks for sharing this link. A varitable treasure trove of Tai Chi Chuan form videos!
My only comment on Brush Knee and Twist Step arm heights is that I don't think it truly matters. Previous forms I have studied have run the gambut from chest height to truly brushing the knee. All are perfectly valid expressions of the same energy, simply moved to differing heights according to the opinion of the forms creator.
Using the same pricipals of movement you can use that arm to do an entire array of things, anything from grasping and offsetting your opponents arm to meeting incoming energy from a kick and offsetting it. What you do with that arm in "combat" will depend almost entirely on the circumstance of the moment, but practicing it depends on what the intent was of the person who created the form you are using.