Voluntary release is one way to judge if a catch has been made. In this play , 2 outs, R2. The guy caught the ball, slammed hard into the other outfielder. Didn't move after the collision. R2 and batter runner circled the bases. Outfielder stayed motionless , ball in glove. Confirmed by U1. Voluntary release or voluntary showing of the ball is not a prerequisite for a catch. Just indicators that can help us determine if a legal catch has been made. A ball secure in the glove a motionless body would also indicate a legal catch has been completed.