Within a shared dream one person must take on the role of the "subject."
The subject's role is to populate the dream space created by the dreamer with projections of his or her own subconscious. If the subject feels that he or she is being manipulated in any way, the projections will seek out and kill the dreamer. The subject subconsciously fills in details that would be impossible for the dreamer to construct.
The method of Extraction takes full advantage of the fact that the subject's mind fills in these details. Oftentimes the subject will try to hide secrets within the dream space that an extractor can then discover and steal.
One can be trained to defend against extraction, this training usually manifests itself in the form of militarized projections.