 If you're not used to it and your lung capacity is in any way compromised, you'll start to feel like you're going to black out. You won't black out, but you'll start to feel a little hazy. So what you do is you breathe in for 10 slowly, like 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 is the length of the first in-breath.