Its luck. You guess the right way and you save it. You can study psychology like Cech has done, you can watch the player's movements, and it all counts for nothing. The best penalty takers in the world use reverse psychology; they change their run-up, they hit the ball differently. To a certain amount, yes it is reflexes. But saving a penalty is exactly the same as scoring one. Guess a side to go, and stick with it.
And no way is Buffon the complete goalkeeper. He is possibly the least commanding goalkeeper in the area of his generation. Great shotstopper, but no communication. And I have seen much better penalty savers. And that's why its down to luck. Jose Reina and Joao Ricardo are incredible penalty savers, but overall they are average goalkeepers, whereas the best in the business, ie. Cech and Casillas, can struggle with penalties.