This electro-magnetic solenoid gets a message from a depressed fuel rocker switch which inturn jolts the magnetic portion of the solenoid to the open position which basically lets go of the fuel door mechanism so you can give drink to your thirsty beast!  Not for the Mk1s, they use a key and lock, how boring!  Sometimes the magnetic portion fails to work due to years of  debris accumulation and a good cleaning just might save you a few bucks!