http://e55lzoju6enqswtvm66sk3uqukcin3377ehgiorplj4d7qyssipls4yd.onion/en/blog/what-is-a-bitcoin-mixer.php
It works by allowing users to pool together their coins, but only with the exact amount of that particular coin. For example, if 150 users decide to merge 0.002 of their Bitcoin together in an address that they control to do one big transaction, everyone can still take control of each of their 0.002 BTC, since the values are all the same.