How does it work?
In a nutshell, Blender 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 Blender to make mixings fast, without the need of waiting for another customers' bitcoins.
Using the mixing code
This approach still has a drawback: there is a probability for a customer to receive his own previous bitcoins. To address this issue, Blender introduces the mixing code.
With the use of this code there is no probability for a customer to receive his own bitcoins or change from it.