Whiplash is actually an injury in the neck that is caused by a sudden jolt or hit. This usually happens in car accidents, while it may still occur if one slips or falls.
A car accident causes whiplash during a rear-end collision. This is where the car is hit at the back and accelerated forward very fast, so fast that even expecting it will still not be able to prepare yourself for the inevitable injuries to come. This is because muscles are voluntarily contracted at 800-1000 miles per second, but in a rear-end collision, the head is contracted at about 300-400 miles per second, with their neck muscles stretched.
When the car accelerates forward, the head is at a backwards position. And with headrests that are not properly positioned or placed, the head can go further back, causing the neck to stretch and our muscles going farther than it actually can. This may stretch and ear tissues, ligaments and joints around the head area, most particularly your neck. And if your neck is over-stretched from the sudden acceleration, you will then again overstretch it due to your head moving forward once the car stops, which can also affect the spine.
This all happens at about 400-500 miles per second, which is way faster than what we can handle, even when voluntarily contracting out muscles.
Here is what happens during a rear-end collision at around 5 miles per hour:

1. At 0 miles per second or once impact has occurred, the car seat moves but the rider or driver hasn’t moved or been pushed forcibly forward.
2. At 50 miles per second, the back of the car seat starts to push the person’s upper body forward, straightening the spine and forcibly moving forward.
3. At 75 miles per second, your body is now at an S-shaped curve, where your whole spine begins to bend, which may cause your ligaments around the spine to become damaged.
4. At 150 miles per second, the upper body has been accelerated forward with so much force that your lower neck has moved back over the headrest. Depending on how the headrest was positioned, as well as its angle and spring effect, the ligaments of your spine become injured at this point. Most of the force are now affecting your shoulders.
5. At 200 miles per second, the force of the car seat finally throws your whole body forward, including your head. Your head and neck whip forward, with everything happening in less than 500 miles per second!

The reason why injuries from rear-end collisions happen is due to the fact that the car’s ability to protect its back bumper, forcing the movement into the inside of the car. Unfortunately, it is the car’s passengers that are affected. If ever the car goes faster, the metal absorbs the impact and the inside of the car (particularly its passengers) are less thrown or forced to other positions. This is why there is no damage during slow rear-end collisions, though it can create greater injury as compared to collisions during higher speeds, it pays to be careful.