http://walletdvl5avrmoxk2fwsf5b76zkmbkoo3r6kqeesq5hsgnq7xhbciad.onion/51-percent-attack
Paraphrasing it here, a miner or miners (let’s call her person A) with 51 percent control of a network could send person B coins while secretively starting a new, alternative block record. In that new record, person A would include a transaction with the same coins she “sent” to person B. Person A can then build off that new record, adding more blocks until it becomes the longest chain.