As I understand it, and perhaps I am wrong, but the footwork for both dances as a man is the same, heel toe, toe, toe heel. However, I was taught that in order to keep the characteristics of the dance, foxtrot has a lower rise and waltz higher. My understanding is that the rise in waltz is to help not only with the patterns but also to reflect true waltz music. The way that I was taught to keep foxtrot rise lower is by pitching the knee forward so that you have the same amount of foot rise but as you would in the waltz but not the same amount of overall rise.
To my understanding, this is done because in foxtrot your overall goal is to have equal step size for your slows and quicks. In order to show a difference between the two dances (besides just timing)you have to apply the proper styling and characteristics as well (which includes rise and sway).
I was also taught that the reason this works is because your lines are mostly created from the leg you are leaving behind, and are created mostly on your first and third steps, therefore the pitching of the knee shouldn't affect any leg lines that are being created.
Now, I would like to conclude this by saying I've only been dancing (instructing) for a year, so if someone is POSITIVE that this is wrong, I would love to know.. however if you are just making and educated guess as I am (from manuals, videos, etc.), then I would love to hear your position on it, but please don't claim it as fact. Thanks!