This is a great tool for hydraulic brake systems, it won't do you any favors if you're running a cable actuated brakes.
In order to use these correctly, you need to loosen the brake bolts, slide this tool over the rotor and in-between the pad and the rotor grab the brake lever and hold it closed so the pads hold the alignment tool in place, then tighten the brake caliper back down again. Then you can release the tool and pump the brakes a couple of times to ensure that the pads are centered and you are good to go. If the wheel is not in the bike straight, this will not work. If the rotor is bent or out of alignment, this will not work. It takes a little to get used to but I've found it very helpful.
Slip these in between the rotors and the pads, squeeze the brakes, then evenly tighten the calipers (using a torque wrench). Perfect everytime. I use to follow the same procedure without the shims which worked about 1/2 the time. Simply a great tool!
While I respect the positive reviews about this tool, my experience with it is not consistent with those remarks. The rotor, itself, is far too thin to allow this tool to do what it purports to do: center the rotor in the caliper. This tool just adds another layer of metal to both sides of a rotor that cannot resist bending when brakes are applied. Each time I've tried to use the tool, I've ended up having to realign the caliper using backlight and eyeball. Sorry to say, this is the one tool I've purchased for the bike that was a waste of money. I've very sorry I purchased it. It might offer some initial centering of the braking system, but that must be followed up with other methods -- so I rated it two stars on the possibility that it might get one started in the direction of properly adjusted brakes. I don't use it at all any more. Waste of time and, as I said, money.
With the caliper mounting bolts loose and wheel mounted, I place the tool's metal arms on each side of the rotor, then squeeze the brake and tighten down the mounting bolts. It leaves me with a nicely centered rotor and the purple portion adds the function of a pad spreader tool. It's a very useful and time-saving tool!