whats actually done in this process?

 

it's like having your brake rotors turned - you are recutting the surface to "make it like new". no more ware marks, smoothens out surface. if you turn the back side of the flywheel you can lighten it. just don't make it too thin or it will warp. there are specs as to what is the minimum thickness for a flywheel.

 

If you want the %100 expected life of your clutch you must do this.