Does anyone know specifics on signal calling using placards?

Tbh most teams would use them as decoys.

It really just depends on the team and how they are you used.. Simply, they are an addition to hand signals used to relay in the formations and plays.. Sometimes, different personnel groupings look to one board or coach whos using hand signals for say a line call while the QB is delivered the play and relays it to the skill position players.. Many are dummy calls..

When teams use hurry up they are usually running predetermined sequence calls that have predetermined signals from the sideline.. Sometimes its a word that changes between series that the QB calls out.. Sometimes its looking to a particular coach or board..