I've tried mimicking his serve sometimes and it actually works very well.
It makes you bend your knees and push up, which gives incredible power and spin on the ball. Also, it gives great deception to your opponent, they have commented that they can't see where it's coming from till it's too late.
The side spin you get is awesome because you tend to hit across the ball.
The only downside for me is it's very taxing on your back, you have to be very, very fit and flexible to do this serve all the time.
Just goes to show what a wonderfully naturally talented athlete McEnroe is.
Returning his serve in the ad court must be a nightmare.