VF or video fluoroscopy is the use of a special x-ray machine to capture moving pictures of the bones as they move.  While not a test that applies to everyone with a whiplash, it can be helpful in looking for instability and/or poor motion in certain parts of the neck.  The test is non-invasive and generally provides more information about the neck or back than an MRI.   This is because most chronic pain problems in the neck and low back are due to poor motion, something an MRI doesn't measure.