Actually it’s pretty well understood.
The human body reacts to CO2 buildup with a ‘gasping for air’ sensation. Nitrogen however, not at all. The air we breathe is 80% nitrogen 20% oxygen, so we aren’t sensitive to nitrogen at all. Breathing air with little oxygen is something well understood as it can happen to pilots of unpressurized aircraft. Here’s a funny example of what happens when pressurization fails. Once ATC figures out he has hypoxia, they order him to descend to 11,000’ (which is usually the point hypoxia starts to kick in) and he’s fine. But while he’s hypoxic, he happily admits he has no control over his airplane and is totally unbothered by that fact.
There’s a thing called a hypoxia chamber- the oxygen % of the air is reduced (not eliminated) to simulate what it’s like being at high altitude without pressurization. Always funny videos there, grown men with oxygen-starved brains playing with a children’s puzzle trying to put the square block in the round hole.
Execution by 100% nitrogen is the most humane death I can think of. The gas is odorless, and as it takes effect the prisoner would experience a euphoric feeling before just falling asleep and dying a few minutes later.
That said, I’m sure they’ll fuck this up somehow- most civilized people have concluded that execution is barbaric and unnecessary, so whoever builds the nitrogen gadget is probably not going to be the sharpest tool in the shed.
And that’s what a botched execution would look like- if you shut off the nitrogen too soon or don’t ensure a high enough nitrogen concentration, the prisoner will be left with brain damage but not dead.