It's been hypothesized that enzymes in your saliva start breaking down the root which makes the kavalactones more readily available. I don't think it's proven, as Dr. Lebot, whose done more kava research than anyone, believes that it could simply be that the extreme force exibited on the root between your molars, is responsible for popping more cell walls and releasing more kavalactones.
I also noted, on my trip, that this strong chewed tanna kava has another obvious aspect to its strength. The size of the chewed up root ball that was used to prepare just one or two shells was huge and appeared even larger than what I'd use for a whole nights session at home. However, they don't even require lengthy kneading to achieve their famous potency. Once it's chewed, it gets put in a bag with a drizzle of water over it, while two guys twist the bag and push on it with stick while the liquids drip into a shell. That one shell is quite strong for such a short time of squeezing. To me this implies a lot of the extraction must have taken place during the chewing, perhaps it's a combination of the chewing and the saliva enzymes.
As for simple sublingual administration, yes it works, sometimes I tuck some in my lip and hold it there. It has a very light effect and would require several rounds to feel anything equivalent to shell. It's good for light anxiety relief though. You also have to consider that when you hold powder in your mouth the small particles mix with saliva and you swallow, at that point you're basically just drinking kava again which might be partially or largely why you can feel kava effects just by holding it in your mouth for a while.
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When you swallow alcohol on an empty stomach you feel the effects come on just as quickly as kava. I believe the same could be said for ghb as well.