That would be the mechanism of "free will" processing input and returning output. We can't see how it works because it's within the "black box" that is the human mind.
A mechanism?!
Do you really mean a mechanism? Because, see, a mechanism has no free-will; it is at best chaotic.
It "processes" input? According to some algorithm? If the processing follows an algorithm, it is not free. If it uses no algorithm, it is random (chaotic). In either case it is not "free".
Now try again, without mechanisms, processing, whatever.
--Boris