there's a couple ways these work.
one is a pure pump n dump. person(s) buy shares, has a stock spam list, sends it out to the list, and dumps it into any buying.
another is company hires a promoter. promoter gets some combination of cash, stocks, and warrants/options on the stock. promoter spams the stock, and as it goes up, he's rewarded (his stock goes up if he hasn't dumped it, and/or the warrants).
the 'better' OTC companies which hire a promoter actually ask said promoter to sort of act as an MM for the stock, similar to the old days (think _reminiscences_). stock goes up, sell...stock goes down, buy. the whole point is to get enough volume so that it seems completely legit, so the company / promoter can eventually get rid of a significant number of shares at much higher prices.
me? i never play these long, and am still looking for a broker where i can get size on real cheap stocks (under 25c) that have gone up, say, from 1/2 c -> 20c in a week or so. i can sometimes short OTCs but can't get size, and they're not the super cheapies.
(EDIT: this isn't unique to OTC companies. it's done on other exchanges, but not as frequent on the others)