www.jetsam.ca has got some info about how the KISS works - a good place to start.
Same as most CCRs - one cyl of O2, one of Dil. O2 is delivered by a regulator with a fixed interstage pressure and flows through a tiny orifice to give a constant flow of O2 at around 0.7-1.0l/min, the idea being that you match the O2 flow to your in-water resting metabolic rate to reduce the mount of O2 addition that is needed.
The original orifice was .004" or summat, which gave a flow of about 1l/min at an IP of around 9 bar (IIRC), so the unit would stop delivering O2 when ambient pressure reached that value, hence a depth limit of around 80m. KISS users then did their sums and found that a smaller orifice (.0035") will deliver 1l/min at an IP of around 11.5 bar, which is close to the maximum that the Apeks 1st stage can delive. Hence a 'depth limit' of around 100m for the KISS even tho it's been used successfully at 115 - 120.
There's a manual bypass button that lets you add O2 directly into the loop to achieve a set point or do O2 flushes. 3 independent wrist mounted PO2 meters let you know when you need to add a squirt of O2 to maintain your PO2 if you're working.
People have now started bolting all sorts of lectric thingummies to their KISSes - Hammerheads and suchlike, similar to the Inspiration. Personally the reason I went the KISS route was that there weren't any electronics, and I don't consider 100m to be a restrictive limit on my diving at present.
K