During constant acceleration your seat pushes you forward with a constant force. The same effect happens (in reverse) when a car accelerates. The force on you from the safety belt changes suddenly as you are flung back into the seat, as it did if the braking also started suddenly the sudden change in the force on you is what causes the discomfort. But there is still a force on you from the springiness in the safety belt, which is under tension, throwing you back into the seat, which is no longer accelerating backwards. The deceleration changes from a constant value to zero in a very short time. However, the car does not move backwards. If the car were to maintain the same deceleration when it had reached zero velocity then it would immediately start going backwards. If the car decelerates at a constant rate, there is a constant force on you from the safety belt, to prevent you hitting the windscreen. 'Jerk' is indeed the correct term to describe both the experience and the cause, which is a (sudden) change in acceleration. Also, there never is any intentional backwards motion here (the drivers are precise, I guess). But then it has to have a proper force 'mirror' as per the third law of motion. I'm guessing it has something to do with the reaction force by the brakes which overcome the forward motion and provide an impulse backwards. I'm curious to know, what causes this secondary jerk backwards as soon as a vehicle comes to rest. But as soon as the train halted, I noticed a secondary jerk.this time in the opposite direction, and it was kind of short lasting. The thing I noticed was that I could feel a force pulling me in the initial direction of motion, as the train slowed down. The thing about such trains (might be irrelevant) is that, being light-weight, their motion somewhat mimics that of cars, and the effects of motion are more apparent as one usually stands in such trains. I travel by subway trains, and I noticed an odd phenomenon. A stopping vehicle (say a car) has an apparent retardation (which may/may not be constant in magnitude) when force via brakes is applied.

Car jerks forward when braking