Double Blind Tasking
Double blind tasking is where neither the viewer nor the monitor know what the target is. The hope is to make certain that the monitor does not pollute the session with his/her knowledge of the target. Double blind tasking is most important in research mode, where experimental purity is an absolute requirement. It rarely ever prevents monitor pollution, though, and is therefore mainly used because of the demands for scientific peer review.
For a list of the problems of double-blind tasking, see also "Double Blind Session".