They can undo transactions. Its a bit more complicated than that, but thats the gist of what control enables. Basically, the first to mine a block has control

They can undo transactions. Its a bit more complicated than that, but thats the gist of what control enables. Basically, the first to mine a block has control over the history since the previously mined block. Because they have superior mining capability, they are more likely to be first to mine a block. They are also able to refuse to accept others blocks, which could effectively fork the blockchain, causing a split head, which results in other other issues. EDIT: just reread, and theres 2 aspects to control. Theres the mining aspect (which i mention above) and then there is the market manipulation aspect (which is to do with pump/dump - unrelated to above).