All Air Force pilots must prove themselves in a controlled environment first. You receive training on how to properly "G strain" to keep the blood in your head which will keep you from blacking out (losing vision) or worse, GLOC (Gravity-induced Loss of Consciousness). This training is done in a centrifuge that spins incredibly fast to place you under anywhere from 1 to 9 Gs. The G strain basically involves flexing your butt and thighs while only breathing short "gulps" of air. Pulling 9 Gs doesn't hurt, but it definitly is exhausting. I personally had to sit down for about 5 minutes and catch my breath after riding in the centrifuge.  Also, pulling Gs can sometimes give people small red dots on their skin.  It caused from small capillary blood vessels popping.  Its only temporary and doesn't hurt, but can be kind of alarming.

If you don't strain properly, you can pass out pretty easily. Here is a link to some footage of the actual centrifuge training.  Its kind of funny to watch people do the "funky chicken" when they recover.  Better to have it happen here than in an actual aircraft.

<a href="http://www.youtube.com/watch?v=lGhydNnqJ5E&feature=related">Youtube Centrifuge Video Link</a>