It works like the non-tablet solutions to writing no screen. It detects when something interacts with the screen using optics or lasers, then uses that to gauge a mouse click. This method is not capable of pressure levels which makes it moot in a lot of cases where it would be used.
I think it could be capable of detecting the pressure level on the pen side. However, I'm not sure it would be that good for a regular screen to endure too mucbh pressure. Plus, it's kind of awkward to write on a regular notebook screen opened at a normal angle.
Overall, the idea is good but it's hard to put to good use on a regular setup.