My first instinct was to go with Alex's explanation, then I thought, "No, Silly, you can't push by using a chain"... (reckon I'd better ease up on the tea maybe). It's obvious, of course - the Chev is pulling the stuck Ford carrier backwards off the road into the ditch, to clear the way for the following convoy of Chevs to get up to the Front. Afterwards, said Chev will disengage, climb out of the ditch, move to the front of the Ford carrier, and proceed to drag it wherever it has to go, since that's the only way it's going to get there.
Really quite simple when you think about it.
