to jackknife a trailer = postaviti prikolicu pod uglom prema tegljaču
http://en.wikipedia.org/wiki/Jackknifing
Jackknifing means the folding of an articulated vehicle (such as one towing a trailer) such that it resembles the acute angle of a folding pocket knife.
When a trailer skids to one side, this is known as a
trailer swing or trailer slew. This could happen on a slippery road surface, often where there is a camber.
This is not the same as "jackknifing" and is not as serious since the trailer moves back into line as the vehicle continues forwards. The driver must be aware, however, that the trailer could slide up against parked cars or the wheels could slide into a ditch.
Anti-jackknife devices ...
The translation workplace 
