I have been doing some hindu squats lately but with feet flat to help loosen up my hips in addition to the shinbox switch. I never do more then 50-100 reps because it always seemed pointless to take high-rep exercises to the extreme. About as pointless as low-rep stuff.
So, I began to feel uncomfortable at around 50. Not that I am winded or something, it just feels like torture. Suddenly I remembered that I must be overbreathing, and all of this deep breathing shit was probably at fault. I once studied a style of hard qi gong that also overemphasized this "suck as much o2 as possible" nonsense.
Anyway, this time when I started to feel uncomfortable I stopped inhaling except the little that came "naturally." I also exhaled extra hard. Within a few reps I felt good again and had my "zoomph" back.
Goes to show you how such a simple concept is missed by so many. Too much oxygen is as bad as too little.