Rear Brake Piston on 2000 question?

I am trying to put new brake pads on my daughters 2000 Mustang V6. I am using a 6 inch C-clamp to try to push the piston back into the brake assmbly but it will not budge. By chance, do I have to turn these pistons like a screw to get them to move back in. I notice 2 grooved notches on the part of the piston the makes contact with the brake pad, and I was wondering if there is for a special breake tool needed to tur the piston so it will retract back into the piston bore??????

PLease Help! I am not a Mustang Guru....

Thanks in advance