A1 to throw in, A2 jumps from FC to BC, catches the ball in the air, and lands in the BC.
1. If B1 tips the throw-in, this is a violation.
2. If no one tips the throw-in, this is not a violation.
I had trouble figuring out the rationale for this distinction, but I think I can explain it in simple terms now. The difference between 1 and 2 is that in 1 the throw-in has ended. Thus, A2 has FC status when he catches the ball. In 2, he does not have FC status (no status during the throw-in), hence no violation.
Maybe I'm the last to understand this subtle point...
