The lorry had a 2 speed rear axle, as he dropped down a gear going down hill, he accidentally hit the axle into low range - equivalent to dropping down a lot of gears at once. As he let out the clutch the revs went through the roof and the engine made a "clattering" noise and stopped.
When the mechanics stripped it down in the workshop it had dropped every valve and holed every piston. Nice one!
I was sat in the passenger seat so had a ringside seat !