Q: What is the theoretical basis of inducing "Barb-Coma"?

Answer:

A barbiturate-induced coma, commonly called barb coma, is a temporary coma induced by a controlled administration of a barbiturate drug, usually pentobarbital or thiopental. Barbiturate comas are used in various settings in neuro ICUs to protect the brain like in persistent intra-cranial hypertension, resistant status epilepticus etc.

Barbiturates reduce the metabolic rate of brain tissue, as well as the cerebral blood flow. With these reductions, the blood vessels in the brain narrow, decreasing the amount of space occupied by the brain, and hence the intracranial pressure.

Also, about 55% of the glucose and oxygen utilization by the brain is meant for its electrical activity and the rest for all other activities such as metabolism. When barbiturates are given to brain injured patients for induced coma, they act by reducing the electrical activity of the brain, which in theory reduces the metabolic and oxygen demand.

Addendum:

Barbiturate (pentobarbital) coma protocol from Vanderbilt University Medical Center  (click here)

Link: http://www.mc.vanderbilt.edu/surgery/trauma/Protocols/VUMC%20Pentobarbital%20Coma%20Protocol.pdf

References:

1. Use of barbiturates in the control of intracranial hypertension". Journal of Neurotrauma (The Brain Trauma Foundation) 17 (6–7): 527–30. Jun-July 2000. 
2. Schalén, W; Sonesson B, Messeter K, Nordström G, Nordström CH (1992). "Clinical outcome and cognitive impairment in patients with severe head injuries treated with barbiturate coma". Acta Neurochir 117 (3–4): 153–9.