HS2 Rotors are thicker than our previous rotors with a width of 2mm instead of 1.85mm.
To set the pistons into the desired starting position for HS2 rotors, you'll need to insert the included brake pad spacer, thick side first, into the caliper. Squeeze the brake and release, then remove the pad spacer. This sets the pistons into the desired starting position for HS2 rotors. Please be aware that you will need to advance the pads to the rotor by pumping the brake a few times, once the caliper is mounted correctly.
The full process can be found in the MTB Disc Brake User Manual.