Without getting too technical....think of pressurised air coming from a compressor attached to the engine. The system works with increasing "G" acting vertically with the body of the pilot. There is a weight on a spring - the weight controlling the access of air into the G suit. Pull more G, and the weight depresses allowing air into the suit. Pull more G, and more air is sent into the suit etc.
I once had a G suit with a regulator with a sticky spring. No air until G really started to bite then "Wham" and it all came on at once which was most unpleasant. There were incidences of the suit over pressurising in flight, but that's why you carry a knife.
As an aside about "G".... We once had a visit to Lossiemouth by a Belgian Mirage Sqn - they brought a twin sticker, and one of the Shackleton rear crew was given a trip. Needless to say, he came back full of it and was later found in the crew room bragging that he had got into some unscheduled air/air with some F4s "...and at one stage we were pulling 8 G!!".
"Yes, but that was metric G and not imperial G".
Recognising the spoof, another chap chipped in "Yes - if you look in the Jag cockpit, you'll find the G meter is dual calibrated, being Anglo French. Metric G is about 2/3rds of Imperial G isn't it?"
"That's right, so "Bloggs" here was only pulling around 5 G."
Exit deflated rear crew chap until he comes storming back in a couple of minutes later - full glass in hand ready to disperse it in our direction.