lol i dont understand how to close my throat >.<
valsava sounds cool though...wanna explain a bit? XD
thanks for the GM info karks
everyone knows how to close the throat, or what I really mean is, the glottis. When you caugh, for example, you do it. You close the glottis, push air out of the lungs, but as the glottis is closed it doesn't work, so pressure builds up, then you open the glottis and the air flows violently out, which is a caugh. You also do it when you go to the toilet to take a dump. You want to do that while lifting. Just remember to let it up and breathe between reps or you'll pass out

It's probably not a good idea if you have blood pressure problems, though.
I don't get why a lot of people present it as a problem that can be avoided by focusing on breathing out slowly, etc. Ask any power lifter, if you want to do a heavy squat, DL, bench, or what ever, you need to take a belly full of air and push, you will lift more. Who cares if you get a bit dizzy after your set? Besides, you let it up between reps, so it's not like you're gonna do a 12 rep bench set without breathing.
The pressure increase from the maneuver can press on some veins, making the heart recieve less blood, which means less blood leaves the heart, less blood to the brain, and you can faint or get dizzy.. but again, you let it out between reps, so I don't get what the big deal is.. unless you got health problems, of course.