It's a throwback from an old Commercial Divers term:
"One Breath From Death DESCO"
I love this therm !
My first diving device was a hand pump with a 50m garden hose and a self-made breathing set with savety-non-return valve and exhalation valve.
As the hose occasionally kinked you had to wait to see if the boy at the pump could eject the kink or maybe he was just too lazy to keep pumping. Well, we were all freedivers and decompression dives cannot be achieved easyly with one person on a hand pump.
IP measurement makes sense if the system has a IP storage tank ,
and to see if the compressor is pumping enough.
Let's assume the IP tank has a volume of 50 l, the compressor delivers 6 bar, the diver is at a depth of 10 m, needs 12.5 l/min and can empty the tank to 2 bar.
If the compressor stops, the diver now has 8 minutes until the pressure in the IP tank reaches 2 bar.
If the diver checks his pressure gauge every 4 minutes, he will notice the drop in pressure.
Whether he notices this solely due to the breathing resistance depends on the regulator.
With a regulator that is specially designed to work with very low IP and a large IP tank,
you can see a point of the IP measurement.