Bro I don’t want to come off as some savant..at the end of the day I appreciate a secondary that can play zone & man at a high level. This is moreso a conversation bc of the level of detail but in the most simplistic way I think everyone could get it is…DB/LBs are assigned to a specific zones as in normal zone coverage…buuuuutttt depending on the routes by the offense, a DB/LB in zone coverage may not be anywhere near that zone when the play ends. For example…a “flat” defender might end up covering a guy in between the hash marks depending on the routes that are run.
Ex: An SLB covering the H/C with a offensive player crossing his face should be able to give a “push” call to his inside backer to pick up the crossing offensive player & that SLB then drops to the middle bc reading the pattern tells you a dig route is likely coming bc the offense essentially wants the SLB to chase the underneath crossing routes bc they got something bigger coming behind
A simple example…A cb in cover 3 or 4 on the weak side with only a single WR has to play that WR as if he’s in man coverage. If that WR runs a shallow dig across the field then he has to go with him & not pass him off & cover grass in the deep 3rd. If he goes with him then he naturally creates the double coverage element that pattern reading adds to a pass defense bc now that same cb is running with the WR through zones where LB’s & other DB’s are.
This is what some coaches would say to each other that understand pattern reading: “zone is man & man is zone” & that one line sort of brings it all together but you’ll never tell/teach a player that
There’s a book about pattern reading by former NFL coach Tom Olivadotti…it’s where I learned it years ago & then got schooled on it up close. I believe a UM DB is on the cover.
Hope this helps.