A crew was working on a single-phase re-conductor project. The digger truck, which was pulling the four-drum puller trailer, had been driven into position to start pulling ropes on a section of line. The puller was at a 90-degree angle to the digger truck to align the drums with the existing poles. The puller then needed to be repositioned. An employee used two safety chains to connect the puller to the winch line from the digger. He then winched up so he could disconnect the puller from the back of the digger truck. At this point the puller disconnected from the hitch and rolled forward on sloped ground, pinning the employee between the digger truck and the puller. He sustained injury to the pelvis and back.