Rule 9.12: -
Players must not obstruct an opponent who is attempting to play the ball.
Players obstruct if they: back into an opponent; physically interfere with the stick or body of an opponent; shield the ball from a legitimate tackle with their stick or any part of their body.
A stationary player receiving the ball is permitted to face in any direction.
A player with the ball is permitted to move off with it in any direction except bodily into an opponent or into a position between the ball and an opponent who is within playing distance of the ball and attempting to play it.
A player who runs in front of or blocks an opponent to stop them legitimately playing or attempting to play the ball is obstructing (this is third party or shadow obstruction). This also applies if an attacker runs across or blocks defenders (including the goalkeeper or player with goalkeeping privileges) when a penalty corner is being taken.
The first ARG player runs at the ENG defence and passes the ball to the second ARG player. She dribbles the ball towards the circle. As she nears it, she turns her body to shield the ball and prevent the ENG defender from being able to tackle her. The Umpire awards a free hit to ENG as a result of this shielding of the ball from a legitimate tackle by the ENG defender.
The attachment shows the relevant slide from the 2015 Umpire Managers Briefing for Umpires.