The technique you are asking about is called a track stand. On a "standard" bike that can coast, you use the crown of the road to go backward and pedal forward very small distances. It looks like you are standing still. On fixed gear bikes, you can pedal in both directions to get the small movement that you need. There are a number of web sites that will teach you how to track stand. Google is your friend.
Watch a unicyclist closely when they are standing still. You'll see that they are moving the pedals backward and forward by very small amounts. The technique is the same as a track stand.
Some people can do it but I don't understand how. If you start to fall to the left, which way do you throw your weight? If you throw to the right, then the throwing it causes you to fall more to the left. If you throw to the left, it works at first, but now all your weight is to the left and you fall over. It seems impossible to me and I can't get it =(
well how to unicyclists balance side to side while not going anywhere?