Any deliberate or involuntary act by a player that disrupts the opponent during a point. Grunting, shouting 'out' before the ball lands, or accidentally dropping your racquet mid-rally can all be called as hindrances. Deliberate hindrance loses the point. Involuntary hindrance means a let (replay). The line between 'involuntary grunt' and 'tactical screaming' is thinner than you'd think.
"He yelled 'come on!' before the ball even crossed the net. That's a hindrance. Point to the opponent."