This article is terrible and does not explain how proof of stake works let alone how it's broken, but links to another (probably better article on etheruem.org). back to studying it for myself, then. I literally have a headache after reading the bitcoin analogy and trying to guess which parts of the analogy I will need to remember for later in the article (hint: none). It would have been simpler to just explain what a nonce and hash is.