'Battle Trance' is an altered state of consciousness that I have currently been working with quite a lot. The name is due to the psychological state combatants may experience when in a situation of which they have to fight.
In such a state, aforementioned combatants are not susceptible to the feeling of fear, or pain. Also often in such a state individuals of a group may act 'as one', a single organism despite the multiple individuals. Frequently, they also may lose their sense of individuality when the state of 'battle trance' is induced.
Personally, when I become entranced my desire is not to lose my individuality, or act as a collective organism (since I often preform this in solitary), but rather more for the loss of fear and pain mentioned previously.
What is the purpose of inducing battle trance outside of a combatant situation? Well, I seem to find it a fantastic way to dispose of bottled up anger or similar negative emotions, thus relieving stress from my personal being. Once the trance is over, one may feel relaxed, refreshed, with a clear mind, or perhaps quite similar to how you feel after centering or grounding. Since I preform this in the home gym, a punchbag is the object on which I release mentioned emotions.
Battle trance can be induced in several ways, involuntarily or not. Purposely, certain types of music, usually heavy and rhythmic -depending on the individual- can provoke it, or when preformed in a group, behavior often used in rituals such as chanting, dance, drumming, stamping of the feet, battle cries and so on can help you achieve the state of battle trance. Specific drugs and alike substances can be used, but for obvious reasons this is not recommended.
Joseph Jordania, an ethnomusicologist and evolutionary musicologist, is said to be the first to coin the term for the state. He proposed that the battle trance was developed via the forces of natural selection, during the earlier stages within the evolution of hominids, as to prevent and defend from predators.
In the context of battle trance within military and other combatant use, a woman named Barbara Ehrenreich stated:
''"The difference between an ordinary man or boy and a reliable killer, as any drill sergeant could attest, is profound. A transformation is required: The man or boy leaves his former self behind and becomes something entirely different, perhaps even taking a new name. In small-scale, traditional societies, the change was usually accomplished through ritual drumming, dancing, fasting, and sexual abstinence - all of which serve to lift a man out of his mundane existence and into a new, warriorlike mode of being, denoted by special body paint, masks, and headdresses.''
Unfortunately little information on 'battle trance' can be provided on the internet that isn't plagiarized from Wikipedia. Books that may be of value on the subject, or shed some light on it, would include:
Blood Rites: Origins and History of the Passions of War
Keeping Together in Time: Dance and Drill in Human History
Why do people sing? Music in Human Evolution
(Specifically pages 98-102)