The blood flow is usually just slowed down. If it is completely shut off, your brain can still function ( depending on your physical properties) on stored oxygen. When this oxygen starts to run out, the brain shuts the lights off, I guess in order to make the most out of the remaining oxygen. You can use your arms and legs, during that brief period of time where no blood is going in to your brain but you did not pass out yet. Given enough time you will pass out.