The stimulation could make the user more aware of what the narcotic is doing, maybe related to the unusual kind of insomnia that especially the strong narcotics and oxycodone in particular can cause . . . it also seems that anything that makes someone feel better is going to help with pain because being in pain is largely the opposite of euphoria, and they will never be able to invent a non-euphoriant strong analgesic because the body does not work that way -- there are people who get a mild bang from naproxen, ibuprofen, ketoprofen, indomethacin, and aspirin for crying out loud, and maybe this is the missing link that makes paracetamol-caffeine-codeine and dihydrocodeine mixtures work a little better than may be otherwise expected.
The synergy of C-Jam with opioids as well as similar findings with other stimulants of course have been known by the medical community from the beginning, usually it is dextroamphetamine or methylphenidate used clinically for this, usually in terminal cancer cases during a rapid increase in narcotic dose to keep function somewhat intact, and the interest in a strong narcotic with lesser impact on vital signs led to the invention of such things as 2,4-dinitrophenylmorphine and the Kirschner Injection aka S E E and Scophedal, which had ephedrine in it. Prescribing Bolivian Marching Powder in tablet form along with morphine, smack, or whatever, was quite common before the First World War and it was used orally to good effect in some cases of advanced cancer and tuberculosis in particular, and the route of administration apparently made it last longer and not set up the up and down on the CNS arousal continuum that insufflation or injection would cause . . . local anaesthetics do have systemic effects as well which can be used for various conditions, such as mexiletine, lignocaine, and others being dosed orally in cases of chronic pain to reduce the trigger sensitivity of breakthrough pain and other things of that nature; there seems to be some resemblance to the way ketamine can be used orally though it is a completely different type of drug.