Obstruction is when a player interferes with an opponent's accessibility to the puck or actively uses their body to prevent the advancement of a player.
Screening or shepherding is one of three subsets of the obstruction foul. This occurs if two players (one of which has the puck) who are on the same team are swimming beside each other and the player who does not have the puck is blocking the opponents access to the puck. The best way for the player who is not in possession of the puck to avoid this call against them is to move a bit further from their teammate or leave some space underneath themselves so that an opponent going for the puck could swim under them. This is not an ideal position to be in while playing, so players should only do this if they may be screening.
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Obstruction is signaled by grabbing both wrists and holing the arms above the surface of the water, then moving the hands in and out from the chest.
As with most fouls, this should only be called if it affects the outcome of the play. Although any player blocking access to the puck is a foul, it should only be called if an opponent tries to get to the puck and fails because of that player blocking them. If not opponents attempt to get to the puck, the advantage rule should be followed and the play should continue.
In this example the puck is knocked under white player #12, who then turns, but does not get possestion of the puck. As he is turned, a player on the black team tries to get to the puck but can't because white #12 is blocking him. The white player is called for obstruction.
OBSTRUCTION (SCREENING/SHEPHERDING)