Why would she be catching on her knees?
Our catcher on our travel team last year (12U) would throw from her knees when taking the ball down to start an inning. The ball would never get above shoulder high, and was practically always a strike. This girl had the strongest arm I've seen by far on a 13 year old girl (and one of my buddies who coaches 16U said stronger than any catcher he had seen at that level also).
I asked her one time why she did it, because she never did it during the game, and she said something to the effect of "just to let them know that they don't want to steal second", with a little sneaky grin.
Her nickname was "Rocket", and it wasn't because she could run fast (although, she did indeed run very fast).