Originally Posted by tical

don't end up like this guy



one of the comments in that vid explains it nicely

The reason this happens is complicated. Long story short, bearing down and holding your breath while straining to the limits of your ability causes your blood pressure to skyrocket and your blood vessels to dilate to compensate. When he dropped it and yelled triumphantly, his blood pressure went back down to normal, but the blood vessels remained dilated. Suddenly a lot less blood is getting to the brain, which decides it's a good time for a nap. Right into the dumbbells.