This theory was first noticed on the T.V. show Ghost Hunters during season 3, when guest investigator/medium Chris Fleming introduced it. The theory states that when using a device with lights on it, such as the K-II Meter, you can ask the spirit yes and no questions and the spirit will answer you by blinking the lights on the meter “twice for yes and once for no”. In theory the spirit will blink the lights either twice for yes or once for no giving you an answer to your question.
SAFETY NOTE: What is the difference when using this device or using a planchette on a Ouija Board? The answer is that there is no difference. As long as you are holding the device in your hand while asking questions, you may allow a spirit to use your bodies electrical impulses to make the device go off. This is why it is extremely important that when doing this experiment you place the device on the ground or on a surface somewhere and not hold it. For the same reasons why we do not use Ouija Boards and so you do not manipulate your results, by having false readings.
Listed below are some of the devices that can be used with this theory:
KII Meter Electro Smog Meter TM-760 EMF Electro Sensor Multi Detektor II