Flexion-Extension views are x-rays that involve movement.  For instance, in the neck, the patient is told to look down and then back as far as possible.  An x-ray is taken at each extreme.  This allows the physician to look for any extra movement that might be occurring between the bones.  Extra movement can mean instability. 

The most important aspect of having this test done is getting the most movement possible.  The test should be done with a leaded examiner who pushes you into as much forward and backwards bending as possible.  Recent research suggests that instability may be missed without a doctor or technician who actually pushes your neck during the study.  If you've had flexion-extension views without this, you may want to have them redone.  You may want to take some pain medications before the test to be able to demonstrate more movement.