These wrist coaches are set up very similar to how football coaches use them. Generally, they have a grid, ie: 1-5 vertically & 1-5 horizontally. In those 25 slots they have the play/pitch they want called. The coach will call out or signal a 3 or 4 digit number (with any two of those digits being "live")which correlates to the desired play/pitch. I have seen coaches use this for calling pitches to the catcher. I have also seen an entire team wear these and use this as their offensive system.
I am a traditionalist too, but these are very effective for two reasons. 1)Less likely a player will miss a sign. 2)Possibility of signs being picked are slim to none (with a band with 25 slots you can have 15 that read fastball, 5 that read curveball, and 5 that read change. You may have to use the same number only once or twice in a game.) And if you think they may have your signs just change the live numbers (odd innings = first two numbers & even innings = last two numbers.)
I am not a fan of the wrist coaches but I can see why some coaches are going to these especially at the college level.