illegal screens ??

[ChrisSportsFan]

Coach Chaney referred to "illegal screens" being set by the St. Joe team. Then today I was listening to the St. Louis Univ vs Marquette game on a St. Louis station and those guys kept referring to "illegal screens" that were continuously being set by the Marquette team. The announcer even made the comment that their coach has been able to teach his players how to do this so it won't get called. Here's my question; if they're "illegal screens" I would assume they'd get called, so what are they talking about? I didn't watch either game so I'm guessing they're screening and then moving in the same direction as the ball handler is moving. ?? I can only guess that the announcers are seeing something and they don't understand why it isn't called. I'm sure they just don't understand a rule. Can anyone shed some light here?