The script reference says:
settweak(scenenum,num,val) - sends a value to a scene tweak. This calls 'void settweak(int num,float val) {...}' in a user scene
so.. when you type 'settweak(find([name],[My Overlay]),[3],[42])' in the console, the function you have gets called, and num=3, and val=42. The function you've pasted in doesn't do very much - when you call settweak(...,[-1],[anything]) it sets amt_dir to -1, which fades out the scene.
You can also use gettweak to get values back from a scene/overlay. The xxx-o-matic scenes are good ones to look at, as most use settweak to set the colour of themselves up. hope that helps.
- Gordon