Not to one up anyone here...
But there is scientific evidence that this actually works to slow the heart rate, release tension and put an athlete "in the zone"...
The idea behind it is that when one moves the lower jaw away from the top jaw, and even protrudes it, it produces a calming effect and allows one to concentrate further.
There are athletes that naturally have done this in the past (think Michael Jordan)...
I had the privilege of being taught by some folks at Vision 54, and this is something that they teach...
This is not to say that some wankers do it simply because they are clowns, just pointing out that for some athletes, there is a real purpose behind it.
Cheers!