http://u3qwdut6yva3iyewthpkmzyfgmt5f7xzoohktgyrafugxzar7dkcq3yd.onion/en/how
In a nutshell, Blenderio has a lot of bitcoins in its reserves. A customer sends some amount of bitcoins and they go to the end of the reserve chain. Afterwards the customer receives the amount of bitcoins from the beginning of the chain. This approach allows Blenderio to make mixings fast, without the need of waiting for another customers' bitcoins.