Straight cast = crooked bone. Crooked cast = straight bone.
Basically a three point mold is utilized to hold a reduction. You are reversing the deformity. So say you have a classic left distal radius fracture from falling on an outstretched arm. The distal piece is tipped dorsally and the apex of the fracture is volar. You would perform a reduction maneuver and then apply a short arm cast or splint with a three point mold. You would hold your left hand (flat palm) at the apex of the fracture. Your right hand would be pushing the distal fracture fragment into alignment. So your hands are pushing towards each other, but at different levels about the wrist. However, you can't hold it with just two points of contact. You need a third point to stabilize the forearm. So initially, you could reach up with your right knee to stabilize the lateral elbow. Now as the cast/splint is hardening you smooth it out and create a nice gentle bow in it to hold the reduction. Of course, there is more to it than that to get a nice looking cast. I would then convert it to a long arm if it was an unstable distal radius to control rotation.
