When a sword is differentially heat treated and polished well enough for it to show through, a hamon is visible as a pattern on the blade. It is a sometimes hazy, delineating line that marks and seperates the two hardnesses of steel on the sword, the harder cutting edge and softer spine.
Not all swords have this kind of heat treatment and these are called 'through hardened' blades. However, some people like the way the hamon looks. Because of this, sword manufacturers sometimes cosmetically alter the blade to show a similar pattern without going the whole hog and actually doing the heat treatment.
This cosmetic alteration can be done in a manner of ways, the most common being bead blasting, wire brushing and acid etching. None of this alters the functionality of the blade and with spring steels, its can be the best option where durability is concerned as any differential heat treatment makes the blade far less flexible.
To add, or not to add a cosmetic hamon. This is one of the subjects that divides most sword enthusiasts. Is it better to try and fake the traditional heat treatment even though it can be obvious it isn't authentic, or is it best to leave the blade clean and shiny?