You hear yoga teachers talk about your breath over and over (and over and over and over...). "Yeah, I get it - focus on your breath. Now stop saying it!" But I don't think I really "got it" until after contemplating things I learned at the retreat. Everyone knows if you're upset taking some deep breaths helps to calm you, but why? Why does simply breathing make things hurt less? Make your heart slow down when you're frightened? Make anger subside? The answer is simple, because you take the focus off of the issue at hand.
We like to think we were made to multi-task, but we weren't. Our cerebral cortex can only handle one task at a time (if you want more detail, here's a great article explaining it: http://www.webmd.com/mental-health/features/why-multitasking-isnt-efficient). Therefore, our brain cannot handle say, acknowledging both a pain in your leg as well as the feel of your breath just below your nostrils.
Most people don't have enough control over their own minds to dictate which one to concentrate on. The pain/anxiety/fear/anger is more demanding so your brain jumps to that one. However, the purpose of meditation is to strengthen this control over your brain. So that in a pinch, you will be able to say "Brain, you are now going to notice the breath instead." By removing focus on that other issue, it goes away. Internally you might struggle - breath, pain, breath, pain, breath, pain - but even removing focus from the pain for a split second makes it less powerful. The longer you can divert your focus from the issue to your breath, the less and less powerful the issue becomes.
I got to test this out last night. I was in one of Jonny's Hot Vinyasa classes and it was a killer (in a good way). After an hour of flowing we started holding some poses - starting with Crescent Moon. I have never been able to hold Crescent Moon as long as he makes us hold it. I see people around me straightening their legs, or going down onto a knee, and I always end up giving myself permission to do the same. Last night it was coming down to the wire. I had a cramp in my bent leg and just wanted, with every inch of my being, to straighten it out for a second. Instead, I focused my attention just below my nose. I began to feel the wind hitting the skin, in and out, in and out. God my leg hurt. Wait, back to breath. In and out, in and out. Ugh, am I going to survive? In and out, in and out. Then the most amazing words in the English language: "triangle pose." I had made it! For the first time ever, I had held the pose without letting my mind give permission to my body to "take a break." Holy crap - there is something to this controlling your mind thing!