Example image using the terminator arm. The pistons on this figure are a part of the main object, not seperate props. The naming is the key. But this will give you an example how it works. The rotation centres (green) are at the hinges where the piston meets the arm parts, the rotation ends (red) are at the end of the piston parts. Both pieces then use "point at" so they know what they're supposed to follow. Left image is the arm at rest, right image is the forearm bent at 60 degrees forward, you can see how the pistons have extended and collapsed to follow the arm position.