The special tool you guys are talking about is for rear disk brakes. The pads hold against the rotor for the parking brake. In order to do this a locking mechanism holds the piston from turning and retracting into the caliper. When the parking brake is applied this lock is engaged. These pistons will not simply press back in the caliper, they need to be twisted back into the caliper.
Some rear disk brake models have a drum built into the rotor and uses shoes to keep the wheels from turning. These models don't have a piston that turns and usually the piston can be simply pressed back into the caliper.
Front calipers won't have these features. Hope that resolves any confusion.
