Well, once you're good at a skill (hitting a golf ball, playing a game, whatever), it's moved into a subconscious part of the brain. Conscious attention to your performance therefore degrades, rather than enhances, your ability to use that skill.
So by distracting yourself with creative trash talk you allow your skill to work unimpeded.
I actually don't see myself getting weaker at something that I've been practicing at/am good at already when I put my full consciousness there.
As I see it, putting your full attention might require more energy, and thus the energy gets depleted faster, so you suddenly lose your control/attention.
Once you've got something perfected, on the performance, you won't need to be so conscious of it, but being conscious doesn't remove the performance capability, either.
Back ontopic, I just somehow CAN'T beat the crap out of Warcraft III. On Medium? No. Don't even think of tryin'. Easy? Not even! Try to make as many military units as possible, and you're 90% sure the enemy has amassed a larger army than yours!