First off, let me give a quick example of a static code block in java:

public class StaticCodeBlockExample {
    static {
        // my static code
    }
    ...
}

The code inside the static block is run when StaticCodeBlockExample is loaded. Among other tasks, this concept is pretty handy for initializing any static properties that might depend on utility classes to setup.

The AS 3 equivalent:

package {

import mx.managers.ISystemManager;

[Mixin]
public class StaticCodeBlockExample {
    public static function init(systemManager:ISystemManager) : void {
        trace("my static code block");
    }
}
}

As long as this class is referenced somewhere in your application, the init method gets called when the application is starting up. Essentially, we have a static code block!