What in the brain has gone awry during sleep paralysis?
Body paralysis occurs during REM sleep to keep your body from acting out your dreams.  You experience sleep paralysis when the mind awakens from REM sleep before the body paralysis has subsided.  I've had this happen before also, along with the "jolting" out of it you describe.  It usually happens when I'm very tired and trying to stay awake.  I try to stay calm but it's not easy as you know.  There is a good description at WebMD.