But it's where the ball is when the KNEE hits the ground , or where a body part goes out of bounds, the 2 have to correlate. Just because the ball crosses the line to gain does not mean it's a 1st if a player's knee hits 1st
You determine when the knee touches the ground, then determine the position of the ball in that point in time. It's not really that hard. Of course, determining when the knee touches the ground isn't always easy to tell.