This happens when one of your senses (the ability to sense where you're limbs are in space without seeing them) is disrupted. Your brain no longer recognizes your limb as your own, and though you can control it, sometimes it acts on its own. The fact that your brain doesn't recognize the limb brings forth the "alien" part in the name.
--the science side of FS
When I was cropping out the rest of this picture I cut it too close to the text I guess.
Nopeity nope.
Nopenopenopenopenopenope
--the science side of FS
https://learning2bdoctor.files.wordpress.com/2010/10/addams.jpg