JD, from the limited picture and description I believe it is a needle dial attached to the spring to measure it's amount of compression. You read off the pressure on a scale depending where the needle is. The scale is in psi as the part is an upgrade replacement for their hardwood
and corian presses, (I couldn't find their corian press on their website), thus they know the diameters > area the force is applied to. Thus you don't need to do the math. Their hardwood press has two different diameter hoops so maybe they have two scales on the gauge.
Their hardwood press is a simple screw style home cheese presses. So you turn it until you've guessed the amount of force is applied, then as whey is expelled, cheese shrinks and force is reduced as the follower and screw above it don't move, just relax, so every so often you would need to add another couple turns to maintain the guessed required amount of pressure. Simple but not great system.
By replacing the threaded rod with this device, you get:
1) An applied pressure reading via the scale on the dial, but this does not directly measure the applied force on the cheese as there are losses due to friction of follower on sides of hoop, but still a good estimate.
2) A more stable force than a simpler screw thread, as as whey is expelled and cheese shrinks, the spring can continue to apply force, albeit reducing as spring's stored energy is expelled. Thus still requiring a re-screw to get back to the correct pressure.
3) A nice small set up.
The cheese presses linked here
including the simpler levered style from a 2x4 wall stud that Wayne uses will apply a (reasonably) continuous force until you are finished pressing. You just have to do your own calculation of area and levered force. Thus you get:
1) No applied pressure force, have to make your own table with different weights and hoop sizes.
2) A continuously applied pressure.
3) A larger set up.
Anyway, that's my guesstimate on it based on the picture and description. The subject of how much force to apply is a whole different discussion/argument as you can see by the several threads on that subject,