The underlying hydrodynamic theory is that: as a vessel is subjected to pressure the structure is compressed. If the structure is made of layers, the layers will be compressed towards each other.
The basic principle of a Compressor Case is that the case back can move towards the middle case, thus increasing water resistance.
Parts of the Compressor Principle can be found in many watches: Bulova and Russian watches with their 2-part case backs; the original Omega Seamaster ProPlof with its crystal retained by a ring allowing it to be compressed with rising pressure (but the back was solid and did not move).
E. Piquerez S.A. (ESPA) filed a number of Patents for a “Fluid-Tight” watch case. As I understand from the documents, the case back screws down against a spring assembly located inside the rim of the case back. The case back is “tight” before fully compressing the O-ring. This allows the case back to move inwards as it is exposed to water pressure at depth. Theoretically, increasing the compression pressure on the O-ring for a more water-tight seal. It also minimizes stress on the O-ring by keeping it at lower compression levels until full compression is needed.
OWC takes the theory to the next level. Our crystal has a retaining ring and the case back is retained by 6 screws. Both crystal and case back move inwards as pressure increases and makes the case more water resistant. Simple but effective.