I think this may be due to weed's modulation of neurotransmitters - (the following is what I'm currently thinking about, from my understanding of what I read, so it may be hazy) the cannabinoid receptors themselves may only be 'switches' to other kinds of biochemistry, and presumably, if serotonin or dopamine are already altered through previously used substances then cannabis has the capability to further modulate their release/inhibition. Of course, research would need to be done to get more specific than that...
I personally first observed this after combining it with an entactogen (I know most of you seem to agree that amphetamines aren't well suited - well, what about partial stimulants/psychedelics?) - I think it was, still, on the comedown, but it propelled me into what seemed to be a full-blown relaxed meditative state (I just assumed the position, with virtually no prior practice, and very soon came head-to-head with the absolute 'present'). I thought it was like a psychedelic trip with not necessarily any visuals. Still, how does this happen? It's reasonable to assume that cannabis itself induced that deep relaxation, as that's what it normally does - and, theoretically, a person could reach this full-blown state easier if they meditate while high - but could modulation of serotonin (or dopamine still, as entactogens alter both) have further boosted this relaxation? (I'm assuming it can't be dopamine as there wasn't any 'reward effect' - this was perfect calmness.)