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.
A few different variants or variations of the complete Compressor Theory have appeared in many watches. Some think that a Super Compressor is a case with twin crowns with cross hatching – wrong. Most Super Compressor cases did in fact have twin crowns and cross hatching, but the crown doth not make the Super Compressor.
Parts of the Compressor Principle can be found in many watches: The Bulova and the 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 pressures (but the back was solid and did not move).
E. Piquerez S.A. (ESPA) filed a number of US 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 gasket. This allows the case back to move inward (very slightly) 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 a ring and the case back retained by 6 screws. Both move inwards as pressure increases and make the case more water resistant. Simple but effective.
Additional information about ESPA Compressor Cases -