For instance, here's how the QB read progression code works:
1) QB randomly chooses a receiver to be the primary target based on the weight of the play and the routes being run
2) The QB waits for the primary receiver to complete a route and then moves on to the next route, waits for it to be complete, etc.
3) Based on the receiver that the QB is staring at, he decides if he wants to throw the ball
How the QB determines the primary WR is here:
http://mfn19.myfootballnow.com/community/3/1361?page=0#8134