Stopping power in denim or any other shirt/suit material refers to it's ability to halt the movement of the lifter while storing energy.
With squat suits, it's the point where the suit will not allow you to descend further (hopefully right at parallel). For the shirt, it's the point where the bar can't be lowered anymore (and rowed to the chest if necessary).
Karin's shirts are great, in that she makes each one for the lifter's supplied measurements. If you have time to wait, she make's the best denim shirts out there. If time is pressed, inzer usually has stock on hand.
Assuming you have several weeks to try the shirt, use it on every max day. Work with boards and work down. Finding the groove is key to any shirt and 90% of the practice is to find this groove.
Oh, and I believe Frantz makes a shirt reinforced with kevlar, for all those South Central lifters.