This pressure on the side of the neck compresses the arteries and veins that circulate blood to and from the brain, causing your target to black out.
Properly applied a blood choke can knock someone out in about 10 seconds, and sometimes even less. This is usually a relatively painless procedure, so when adrenaline is flowing your opponent may not even realise that he's been fully checkmated until it's too late and he slumps unconscious.
Classic blood chokes include the properly applied rear naked choke (2nd picture above), the front naked choke (3rd picture), most triangle chokes and many collar chokes.
In a blood choke you attack the sides of the neck!