According to Yaqui indian sorcerer Don Juan Matus (see accounts of Carlos Castaneda), the Assemblage Point is that aspect of consciousness where energy lines of perception are brought together and 'assembled' into one place for the purpose of their perception.
Explanation of Castaneda concepts in simple terms for those who aren't familiar with the accounts of Castaneda's experiences:
There are lines/frequencies of perception that run through the universe and have their corresponding parts through the human psyche. Most humans only consciously recognize about 2 or 3 of those lines of energy/perception within their own consciousness: that of solids, liquids and gases. To comprehend or perceive these frequency lines of energy in the universe, a bright glimmer of Awareness that is said to exist somewhere behind the shoulder blades within the aura (a.k.a. the Assemblage Point) allows those particular energy lines or groupings of energy lines to pass through it. Different groupings of different frequencies can create different types of perception. Matus makes mention of the process of 'assembling whole new worlds' in this manner. Put into shamanic sorcerers' terms, rising on the planes or the raising of one's consciousness can be thought of as an act of moving the assemblage point to encompass a different range of frequencies for perception.
Something that Juan Matus (as documented by Castaneda) mentions is how amazing it is that most people are able to learn to keep their assemblage points so stable around only a limited grouping of energy bands, consequently all sharing in the same perception of reality. He considers this to be a supreme act of perceptual acrobatics, as well as simultaneously a huge disappointment. It is disappointing in that as a result, humans mistakenly believe what they perceive in that narrow grouping of energy bands to be ALL there is. The 2nd disappointing and amazing thing to him is how most humans believe that the world is how they think it, rather than seeing it how it REALLY is. According to those teachings and observations, most people have a preconceived set of views about how the world works that they project onto their environment, and they are constantly shaken up and proven wrong whenever the world and reality refuses to conform to their limited views.
Similarities of this form of Sorcery to Western magick practices:
Juan Matus speaks of a 'right' and 'left' side of awareness. His description corresponds very well to Ruach and Yechidah (of Hermetic Qabalah), or physical normal awareness and true spiritual awareness (that which most people are unconscious of and learn to access through magickal study). There is also reference to the 'Eagle,' which is described in a similar way to Native American concepts of the 'Great Spirit' and Qabalistic concepts of the 'Point Source' or 'The All'. His concept of Recapitulation of past events in one's life has almost Buddhist elements to it, similar to the Sammasati meditation; the process of bringing up all the memories of childhood into consciousness. (The purpose being to release the built up energy tension of past events and to create a freer flow of life force through the individual as a result.) Psychoanalysts discuss similar methods of regressing the individual back so that he/she can remember the past and resolve those issues. There is reference made to the right and left sides/polarities of the aura itself as well; the right side being described almost exactly like the Pillar of Severity and the left side being like the Pillar of Mercy in Qabalistic meditational/magickal practices.
For anyone interested, I highly recommend a thorough study of Carlos Castaneda's books. These are supposedly non-fiction accounts of his experiences studying with a group of sorcerers of the American southwest and Mexico (whether or not any of it really happened is irrelevent to me - it's still inspirational reading). His later books such as "Fire from Within," "The Art of Dreaming," and "The Active Side of Infinity" are by far the best. Another useful text is "The Teachings of Don Carlos" by Victor Sanchez; a condensation of the basic concepts of Castaneda's books put into a workable format for practice.