For once, something measures you instead of asking you.
Every other exercise relies on you judging your own effort correctly. The gauge does not: squeeze and the pressure loss visibly slows. Squeeze maximally and the needle travels backwards. The cylinder becomes a dynamometer for the pelvic floor — and that is what this programme trains against, twelve weeks from 100 to 300 mmHg.
Your pump may not rest at zero. The app asks for that zero point once and counts every pressure from there, so the ladder below is actual vacuum; what you dial is your zero plus this. Conversion: 1 inHg = 25.4 mmHg.
About the top step, plainly: clinical sources put the ceiling near 250 mmHg¹, rehabilitation studies work around 200 mmHg², and approved devices carry a pressure-limiting valve for exactly this reason. 300 mmHg sits above that. It is in the programme because it works in practice — but only at the end of twelve weeks of habituation, and only while the skin stays clean.
The cylinder holds a fixed volume of gas under vacuum. Anything that changes how much of it the penis occupies shifts that volume — and the gauge shows the shift immediately. A pelvic-floor contraction draws the root of the penis inward and changes exactly that; the pressure loss brakes, and with a strong contraction the movement reverses. What you see is not a translation or an estimate but a direct mechanical consequence of your squeeze.
In practice that means you do not need a biofeedback device to know whether a kegel landed. The pump you are already using tells you.