Taking aside whether connecting a mystical energy to a biological basis is a good idea or not, how is it supposed to work? I know that generally the more midichlorians, the more Force potential. Is it explained in one of the expanded universe books?
They are magic. It is all just magic. Star Wars is fantasy. I'm not sure what you are looking for? A fantasy explanation?
Midichlorians are the explanation for the force. It doesn't really do deeper than that. I'm sorry.
If you really want to travel down this dead-end of illogical paradoxes, I will refer you to the wookieepedia
Let's think about this for a second. Let's say we have a microorganism that happens to have a natural electromagnetic property that allows them to be influenced in a very very tiny way by electromagnetic fields.
Now let's say that a person becomes infested with a lot of these microorganisms but they just happen to be non-harmful to us...
Now every time there is a electromagnetic field (the earth, a magnet, a piece of machinery, a ball of plasma)... the microorganism moves in response thereby moving our body or even causing some kind of reaction.
This is not unreasonable, and certain on the road to describing realistically a kind of force sensitivity.
So really, as much as I prefer a Zen like explanation to the force and Jedi... this could be plausible in a theoretical sense.
Midichlorians are the bacteria or organisms in the body that provide the connect to the "force". These bacteria often would tell how much control over the surrounding world one has. Take for example this:
Anakin Skywalker had no father. Darth Plagueis created him using Midichlorians. This is why he was so prone to falling to both the light and dark side. The Midichlorians tipped him off balance.
I wish I could find the reference, so feel free to edit this answer and clean it up, but midichlorians were supposed to be micro-organisms which are attracted to people who are strong in the force. They don't generate the force, they simply identify where it is strong. And as it's possible to detect the midichlorians, it's possible to use them to identify strong force potentials.
Why Qui-gon couldn't simply use his force powers to detect that the force was strong with Anakin the same way Yoda and Obi Wan did with Luke is never explained though.
Midichlorians are a type of organism that have a closer link to the most fundamental elements in the universe. All living creatures have a certain amount of these organisms in them. A being that has a larger amount of midichlorians has a closer link to the universe and so is better equipped to manipulate the universe. The ability to defy gravity and to see a certain distance into the future are typical.