http://whirtorrgetftvz4g466sjqkegtyi35bjl4bvotfkfossunf5my4x6ad.onion/blog/what-is-a-bitcoin-mixer
The centralised mixers are pretty simple to understand. In their case, the users just send the bitcoins to the mixer and receive back different bitcoins, paying just a small fee for this service. Yet, there needs to be a disclaimer connected to the centralised mixers and that is that sometimes, they do not operate completely open-source, which means that they might keep track of the incoming and outgoing bitcoins and addresses connected to them.