Chipper was raised on a bottle jack so some maintenance work could be completed whilst on-site. A sprayer started to reverse back and as he reversed he came back outside of the work area and hit the chipper, knocking it off the bottle jack. When this occurred, the fitter was lying on his side with his right arm under the chipper. When the chipper came down, the wheel arch made contact with the IP's shoulder, but they managed to pull away.