Sand carving is where an image or design is cut out of a mask which is then placed over the glass or sometimes it is cut out after the mask hadbeen placed onto the glass. The rest of the glass is then covered to protect it and the areas which are cut out are blasted with sand in a sandblasting unit before the mask is removed and the glass is then cleaned.
Sandcarving or sandblasting is good to engrave large areas in less time than hand drill engraving although it can be quite flat.
The engraver can produce multiple layers of depth by engraving different areas for a longer periods of time carfully picking out various sections of the mask to reveal a new area to sandblast. Generally the first areas picked out will be the deepest.
Shade can also be achieved by sandblasting areas in much the same way as an 'Air Brusher' would shade a spray painted image.
Comercial engravers who sandblast designs onto glass will generally not spend much time sandblasting as the grit used can be very expensive and sandcarving deep takes more time, therefore the engraving will not be very deep, in fact it will be hardly noticable to the touch.
This way mass produced items can be made cheaply and quicky.