This was my $.02 from the Spring.
I haven't seen nor read anything. I used to signal our plays on Friday nights when I coached. I sat down with our QBs and we worked out a system that was easy for them. For example; our bubble screen was called "Boston 60/61". We used a number tree on my body, top of the head was zero and worked our way down to 9, which was both knees. For the "Boston" part, I grabbed my ass cheeks, a la Boston Butt. We ran a Guard trap from the gun that was "Georgia 80/81". I hiked my leg like a "Georgia bulldog". Our tackle trap was "Texas 84/85". We used the hookem horns sign. The players loved it!
As for the cards, if the QBs are given input with it, it does run quicker and more efficiently.
When I was a walk on here, Coach Homer Smith had the QBs use hand signs to relay a play or audible to the rest of the team. We had the tug of the facemask for Fade, tug of the collar for a "hook" route, we had the signal of smoking a cigar to signal a "smash" route, we gave the "okay" signal to declare "opposite" play or side was being run.