Pump and dumps target low-float stocks, micro caps, and small altcoins, where a modest amount of buying moves price a lot. The promoters are positioned before the hype starts; everyone who buys after it is their exit liquidity.
It is market-manipulation and illegal in securities markets. In crypto the rug-pull is a close cousin.
Example: a penny stock trades 50,000 shares a day. A paid promotion sends it up 300% on 20 million shares in two days; a week later it is below where it started.
Related: rug-pull, market-manipulation, float, bagholder, dump