<Li> Electronic brakeforce distribution varies the pressure to the rear brakes during heavy load or hard braking, enhancing driver control . </Li> <Li> Tractors used to be fitted with a lever in the cab to operate the trailer brakes . The vehicle could be slowed down or stopped using the trailer brakes only . Theoretically this was a sure way to prevent jackknifing, but this lever was often the cause of jackknifing in a roundabout way . Frequent use of the trailer brakes alone caused them to overheat and fade while the tractor brakes remained fresh . In the event of an emergency stop, the driver would go straight for the foot brake and the truck would surely jackknife because the tractor brakes would lock while the trailer brakes would be ineffective due to previous overheating . </Li> <Li> An alternative to having a trailer brake lever in the cab is to fit the trailer with an electromagnetic brake . </Li>

Where does the term jack knife come from