I've been using this tool for years and have always had great results. Works well for both single and double pistons. Just put something flat in the caliper (like an old brake pad) to push against and it works great. Well worth the $10.
This tool is completely useless and a waste of money! The best tool to compress the piston in a brake caliper is a set of 16" craftsman channel lock type pliers. this tool is a design that was come up with to sell that's it. even a six" c clamp works a lot better. i tried to use it one time and it has been sitting in the bottom of my box since.
in most cases you can stick a small prybar or large screw driver in the hole threw the top of the caliper into the grooves in the rotor and pry back towards you and get them to compressed before taking the caliper of the car.
For the money, not bad. I would recommend that if you're going to do more than 2 sets of brakes you get a better tool. The knob on this one came off after I did the second set of brakes!