NCAA points of emphasis document:
"Knowing what constitutes a legal screen is imperative. Screeners must establish a legal position without causing contact. Delaying or preventing a player from reaching a desired position is the purpose of a screen and while contact will most likely occur, the screener cannot cause the contact. After setting and maintaining a legal screen, a screener is permitted to roll. This roll/movement may also delay or prevent a player from reaching a desired position, and when done without holding, clamping or creating other illegal contact, is legal. Lastly, defenders cannot be permitted to hold, reroute, or push screeners, or push through legal screens."