A parfocalising ring is as simple as it is useful. It can be used to set the eyepieces of your binoculars and your camera to be sharp by placing each one in the eyepiece until it hits the eyepiece. It is not necessary to twist the focus ring to get a sharp image in both accessories.
The 1.25" package includes an allen wrench for attaching the caterpillar screw on the ring.
The 2" version is made by Baader and has a clamping ring design.