Here is a very short, but interesting letter to the BSD from Mr John Browne.

http://www.dowsing-research.net/blog_extracts/BSD_No265_1999_p331.pdf

It contains a survey of the pendulum responses of nine dowsers. For those unaccustomed to the use of the pendulum in dowsing, there are typically three actions it will demonstrate. The first is the neutral response, when the dower is posing the question. In many accounts that aim to teach how to use the pendulum, it is suggested to let it swing to and fro along a line. In response to the question, the pendulum will give either a yes or no response. Again, it is commonly suggested, when learning to use the device, to specify two differing movements that can be easily distinguished. For instance, gyration to the left, or to the right are often suggested, the operator being free to assign meanings to either. In addition to the three basic movements, there can also be a fourth action, distinguishable from the others, here it is referred to as the idiot response, because it occurs when the question is not well posed, and neither a yes, or no answer is appropriate. 

In the table, it is interesting to observe that each of the dowsers exhibits a unique set of the four responses, with no two dowsers having the same set. Perhaps another interesting observation is that for some, the neutral position is a stationary pendulum, but none of the remaining three actions has this stationary signature, ie the response to the questioning is always an active one.

As alluded to above, when setting out, the novice dowser may specify the meaning of the pendulum’s movement, or alternatively they might just accept any movement the pendulum exhibits to their request to demonstrate the response to the four situations.  Given the personal nature of the reaction, it seems again that the dowsing reaction is mediated by (unconscious) mental processes, which as has been commented on previously, is subject to a type of mental programming.