Illegal screens are sometimes difficult, especially when the screen comes from behind the defender.
There are several key things to determine if a screen is an illegal screen. Rule 4.40 Article 2 states that the screener may face any direction, time and distance ARE relevant, the screener must be stationary, except when both are moving in the same direction and path, and the screener must stay within their vertical plane (Rule of Verticality).
First, you have to decide if the screen is coming from inside the visual field, or outside the visual field of the defender. If it is outside the visual field, the rule is that the offensive player must allow “one normal step backward without contact.” If he is inside the visual field, the screener may be “anywhere short of contact.” To be able to correctly rule on a screen, these things have to be determined before there is contact.
In this play, the screen is set outside the visual field of the defender. The defender is not allowed one normal step before their is contact. This example is an illegal screen.
Click here for another example of an Illegal Screen set from behind.