You definitely jerk forward,u feel it and somebody at rest on the groud sees it as well.However,keep in mind that when a car is decelerating,its velocity vector (and the coordinate/position vector) is pointing forward (in the direction of movement),but the acceleration vector is pointing backwards (that's the definition of deceleration:negative acceleration due to opposite sense of the vector acceleration wrt to the position and velocity vectors) which means that the car (and you with it) experience a force pointing backwards.Here steps in the first principle of Newton which tells us that bodies tend to keep their motion tendency,viz.when they are rest,they tend to remain at rest,when moving in direction,they tend to move in that direction,in the case when external forces would alter their movement.In the case of the car,your tendency is to go forward (together with the car,of course),yet an external force (the one from the engine of the car) will alter your movement,and here the first principle comes in and says that an inertial force will act upon your body in the direction of movement,exactly to compensate the effect of the external force.It's acceleration is exactly the same to the one the external force is giving,but,obviously,the sense is opposite.
Daniel.
