Yes, I know how they work. The point about pressure is that the shape of the watch has to be such that the sensors are pushed into your wrist. If ambient light gets between the sensors and your skin, it can't get a reading. If the watch back were completely flat, it would have a very difficult time getting a good reading.
Many other watches have the sensors protruding in some way, and they work best when the band is tight enough that those sensors will actually leave a bit of a mark in your wrist after wearing them.
Garmin's also protrude. In fact on the new 235 they abandoned the slightly protruding sensor and sealing rubber light ring in favor of a more aggressive protrusion.