When a car hits you in a rear end collision, the car initially has a momentum going in one direction. This causes your car to move in the same direction that car was moving even if you were at rest. So, for conservation of momentum, you initially have momentum going in the east direction for example, after the collision, you will have a change in momentum which causes you to have a velocity in the west direction. This is because you are initially at rest and then there is a sudden change in velocity so when you speed up, that momentum causes you to move backwards. If you don't have a properly adjusted neckrest you could may experience whiplash.
