When you practice tasks, you get better at them, when in the same (or relatively same) situation as the practice. For example, if you get really good at playing chess wearing a tshirt and shorts in the snow, then you'll probably not be as good at chess in 40 degree heat.

Same deal with performing under influences of drugs. Up to a degree, if you do something more while drunk then sober, and get quite good at doing it while drunk, then you it won't necessarily translate over completely to when sober. Of course, if you're totally wasted, then it's all bullshit.