YLUSA.com Rules Tip

Interference or Illegal Screen?
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By Eric Evans
February 4, 2002
Rarely does a game at any level not have at least one or more interference or illegal offensive screen (illegal pick) calls. Many times, however, these two technical fouls are confused with one another. The key to keeping them straight is to remember that interference most often involves the team on defense while illegal offensive screen is always only called on the offensive team with possession of the ball.
Interference Caused by the Defense
Interference (not goalie interference): A player may not interfere in any manner with the free movement of an opponent except when the opponent has the ball or is within five yards of a loose ball (either on the ground or in the air).
Interference most often occurs in front of the crease when a defender interferes with a crease attackman or a cutting middie by hindering their free movement or checking their stick…all while the other team has the ball.
In many instances a verbal warning "let him move blue!" can prevent a flag. Yet if there was an advantage gained by the defense—or if the defender does it again—you must throw the flag.
But interference can happen anywhere on the field. A long buddy pass invites interference (player checks an opponent’s stick before the ball is within five yards, etc.)
Illegal Offensive Screen: Middle Name Says it All!
An illegal offensive screen can only be called on the offensive team; "offensive" is written right into the rule. Remember, a team goes on offense when it secures the ball regardless of position on the field.
Once again, advantage/disadvantage can be applied, just like with interference. In some cases, an illegal screen by definition may have occurred, but a warning or a no-call might be more appropriate than taking the ball from the offense.
Most illegal screens will, of course, occur in the offensive goal area, but can also occur (infrequently) deep in a team’s defensive end as soon as a D-man scoops up the ball. He may be 100 yards from the opposing goal, but he’s on offense at that moment and if his fellow defenseman runs into a chasing attackman and blocks him-- or "picks" him--it’s an illegal offensive screen.