EX2 ND filter ?
How is it incorporated ?
Just a guess - there is probably some mechanism which slides (or rotates) the filter in and out of the optical path, somewhere within, or perhaps behind, the lens.
Since such a filter could affect the way light is refracted on its way through the lens, it's also likely that when it is not in use, a similar thickness of clear glass is moved into place, ensuring consistent focus etc.
Many less sophisticated compact cameras already incorporate such a filter, which serves a similar purpose, that of avoiding very small apertures such as f/16 or f/22 which would be unacceptable due to diffraction.