Alcohol lowers inhibition. As in, it doesn't make you do something you normally wouldn't want to do. It makes you do things you normally wouldn't do for whatever reason you thought it was wrong. Basically, it takes away that inner deciding point in your head where you weigh out the negative and positive aspects of your potential decision before acting, and you just go for it with no inhibition.
With this in mind, it becomes easier to legally decide whether a choice made while drunk is coherent or not. Free will, in its purest and most unadulterated form, is involved with alcohol. This is why drunk people say the things they always want to but never do normally, and do all of those stupid things they normally wouldn't want to because it's dangerous. Their free will is going wild.
The point where they can't do anything, or don't even know what's going on, is too drunk. This is what people think 'blackout' drunk means. But they're thinking of passed out, which is a totally different thing. All 'blackout' means is a period of alcohol induced amnesia.