[Rev.Ref63]

15.2.4 Crossing the Center Line
15.2.4.1 Encroachment into the opponent’s court with the foot, feet or
hand(s) is permitted, provided some part of the encroaching extremity
remains in contact with or directly above the center line, and there is
no interference with opponents. It is not a fault to contact the opponent’s
team court with the hair.
15.2.4.2 Encroachment into the opponent’s court with any part(s) of the
body is permitted, provided some part of the body remains either in
contact with or directly above the center line, and there is no interference
with opponents. In addition, completely crossing the center line with
the foot, feet or hands, or encroachment with other body parts must not
present a safety hazard to opponents.

One of our players was called for crossing the center line. Can someone explain this rule to me in layman's terms?