Looks like they used my question I sent in
I noticed you always used the knee stance up until this year. Why did you make the switch? (Mark)
Using the knee stance puts quite a bit of pressure on the kneecap and all those years of using the stance were starting to catch up with me physically. Coming up into the box stance has been great for me. Of course, Ive used the stance before, having trained many umpires to work that way. The main thing isnt really what the legs are doing, but that the body has positioned the eyes to see the pitch well. Thats why youll see different stances. Different people will find their comfort zone in different ways.
http://worldumpires.com/askUmp.htm