I can't tell you how it works, but I can tell you that it does work. I stayed at a hotel on the 6th floor all last week and took the lift multiple times a day. It only ever tracked stairs when I walked up them. I did take the escalator at the airport and wish I would have checked more carefully (I wasn't thinking about this feature) but I don't think it counted.
Excellent, thanks! I'll have to test the escalator but if one works I expect the other does.