What is the ideomotor effect?
The ideomotor effect is what is seen when someone uses a device such as a dowsing rod, or when a group use a Ouija board planchette. The object will move slightly due to subtle unconscious movements of the holder's hands which can lead the person holding the object to believe it is moving without human intervention and thus indicating some kind of external force.
This external force is often then interpreted as an external agency such as spirits, ghosts or gods (the phenomenum is sometimes called divination) or an unknown scientific force.
In the case of dowsing rods, these movements are taken as a sign of whatever substance they were looking for when in reality it is more likely that the user has experience of where to look from other clues and is subconsciously moving the object to correspond with their own pre-determined biases.