From http://www.gemsinfo.com/hthp.asp

Clarity Enhanced Diamonds (CE) are totally natural Diamonds treated with a New Clarity Enhancement Process. This new revolutionary process optically eliminates "feathers" (inclusions), which are naturally occurring imperfections found in most diamonds.
The new Clarity Enhancement Process (HTHP) is a complex process that involves heating the stone while it's under pressure until the desired alteration of color and/or clarity is evident.
This treatment is only performed on certain diamonds.
The clarity enhancement is permanent and is not visible to the naked eye.

The result is a more visually stunning diamond that you can purchase for significantly less than a similar-looking, non-enhanced diamond, where only you and your jeweler will ever know that you own a Clarity Enhanced Diamond.

When light travels from one medium to another, it either changes its course or reflects in a different direction.
When light attempts to pass through a non-enhanced diamond that may have typical feather inclusions, the light hits the feather and reflects off in any number of directions.
The more feathers, the more the diamond appears to be non-transparent.
With a CE diamond, the light passes THROUGH the natural inclusions, because the material used for the enhancement has the same optical characteristics as the diamond.
The beam of light "thinks" it's still traveling through the same material (diamond) and continues its original course.
The amount of material used to fill the feather is so minute that the Gemological Institute of America (GIA) has reported that the clarity enhancement process ADDS NO WEIGHT to a diamond.
This treatment method is permanent and normally does not create special care requirements. Clarity Enhanced Diamonds are normally GUARANTEED FOR LIFE by the manufacturer.
CE Diamonds are natural diamonds from deep in the heart of the earth which have been technologically enhanced to become as brilliant, clear and beautiful as modern science can make it.
The only things likely to compromise the integrity of your diamond's enhancement are the flame of a torch or boiling acids and bases.
In the unlikely event your CE diamond should lose its enhancement, usually the manufacturer will restore the enhancement of your diamond.