The ball handler and his defender are moving.  So, no teammate of the ball handler is allowed to move in to try to set a screen on that defender.   It is still an illegal pick and therefore an offensive foul if the player setting the screen (the screener) alters that defenders path or inhibits his movement, even if the screener does not make contact with that defender.  Also, the ball handler must understand that he cannot go in motion before the would-be screener stops moving. Often times, the ball handler actually causes the foul by moving early toward the would-be screener, but the screener is charged with the foul.  The screener must have the screen set fully (that is, he must be stopped and motionless and his feet no more than his shoulder width apart), before the ball handler should move toward the screener, which will have the effect of bringing his defender into the legally pre-established screen.