It's otherwise known as Dr. Strangelove Syndrome, which should give you an idea of its symptoms. One hand appears to act independently of the rest of the sufferer's body, performing complex actions that are often in direct opposition to the person's intention. Your right hand, for example, might shake that of your girlfriend's father, while your left hand reaches around and gives him a cheeky pinch on his buttocks.
Due to its willfulness, sufferers tend to associate the hand with a specific personality, which is usually that of a total fucking dickhead.
Will it get me laid?
For a shy person, a rogue hand is an invaluable gift. Say a girl has been throwing you some subtle "bone me" signals that you've been too damn introverted to notice or believe. But your hand has noticed, and you watch in awe as its response leaves absolutely no room for doubt in the girl's mind that you are interested in commencing intercourse. To clinch the deal, you can now give her a line such as "Hey baby, do you know what my hand will be doing in 20 minutes? (cock eyebrow suggestively) Me neither!"
If that doesn't work (and let's face it, possessed limbs are often not the best judges of subtle moods), you've still got someone else's hand on the end of your arm. If you can persuade it that you love and respect it very much, and that your erogenous zone is not going to stimulate itself, you don't really need to get laid at all.
How do I get it?
Alien Hand Syndrome is caused by a particularly hardcore treatment for epilepsy in which a "doctor" surgically separates the two hemispheres of your brain. Due to its suicidal insanity, this operation can only be performed by a cackling madman in a lightning-lit castle at midnight. The lightning is important; on clear nights, a nurse is employed to switch the main light on and off really quickly.
Is it worth it?
It's your call. While the condition is rarely fatal, it can be very inconvenient. Are you a teacher? Better be careful. It only takes a few instances of "I swear it wasn't me! It was The Alien Hand" before the parents' association pays to have you arrested with unnecessary force. On the upside, you'll most likely get laid in prison.