First off, there's no such medical condition as "whiplash." This is just what normal people like you and me call it; it's a term that goes back about a century. A doctor would call it something with a lot more syllables, like a cervical hyperextension injury. The advantage of a long name like that is that it tells you something about what it is and what caused it: It's a case of extending your cervical vertebrae ("neck bones," to you and me) into a position where they shouldn't have been.
Let's go back and look at a slow-motion instant replay of that fender-bender accident. There you were, sitting still in the driver's seat (or maybe the passenger's seat), minding your own business. Then your car starts moving forward unexpectedly, but nobody tells your body that it should be moving forward along with it. The first clue your body has that it should join in on the movement is when your car seat hits the back of your torso, shoving the lower middle of your spine in the same direction that the car is moving. After that it takes a tiny fraction of a second for the upper part of your spine, where those neck bones are, to catch on to the fact that it's supposed to be moving too and during that fraction of a second your head snaps back, bending your spine into an s-shape not usually found in nature. This puts a strain on the soft ligament tissues that hold your spine together. Before even a single second has elapsed, the neck realizes that the lower spine is moving forward without it and rushes to catch up, correcting the s-shape. Everything's back to normal -- maybe.
Nobody knows exactly what part of this s-shape -- a position that your spine really only occupies for a fraction of a second -- causes the symptoms of whiplash, but it's clear that your body wasn't intended to deal with the sort of stress that the rear-end collision puts on your ligaments. (This sort of fraction-of-a-second spinal strain can be caused by other things in addition to rear end collisions, but in our modern world of ever-more-frequent automobile travel, it's the most likely scenario.) What we do know is that a few hours or days after the accident the whiplash symptoms can start to arrive and they can do everything from putting you through excruciating pain to making it difficult to move your neck. They can give you headaches, dizziness, fatigue and a ringing sound in your ears. Or maybe you'll luck out and none of these things will happen. The human body is a funny thing.
Since it'll probably be a while before rear end collisions can be prevented altogether (although self-driving cars and cars with radar-based collision avoidance are moving us in that direction), the best thing that automakers can do in the short run is to redesign car seats so that the s-shaped spine typical of these accidents isn't likely to happen. And that's exactly what they're doing.