A chop block is when a player is already engaged by one blocker high and another cut blocks him low. It has nothing to do with recently blocked or angle of the cut block. It is a fairly simple call to make. Here are some examples to help you understand better from our morally superior friends in Athens who you would think would never do such a thing.
http://www.youtube.com/watch?v=cq3MkoRGoyU
http://www.youtube.com/watch?v=3f6L1XlSTrE
Where did I say that we've never done it? Here's an example of your team doing it and not being called for it. Thornton had to be helped off the field afterwards and missed the SECCG with Bama because of this.
http://youtu.be/QpTN1TaVK2Q