In basketball and lacrosse, the offensive player setting the pick must remain stationary at the moment of contact with the defender, and allow the defensive player a "reasonable opportunity" to avoid the screen; a screen is illegal if the screener moves in order to make contact, and obtains an advantage; the result is an offensive foul in basketball and a technical foul in lacrosse. There must be illegal contact for a moving screen to be a foul; no illegal contact, no foul, no matter how much moving the screener does. If the screener holds, leans or moves into the defender to cause contact, this will result in a foul on the screener.
The objective is to maintain fair play by prohibiting screens that create an advantage for one team over the other; the screener must remain stationary during contact with the defender to avoid a foul.