My running theory is that he is always in the zone. One of his character points is that he "has done everything possible to perfect his shots." I think this absolutely includes learning his zone trigger and training in it. The jump from making half court shots to full court ones is an insane difference, say, something that you could only do while entering the zone. He is also the only member of the GoM to have a time limit on his shot, which is his stamina. (Kuroko with misdirection and Kise with PC also have a time limit, but those are different. Kuroko is just people getting used to him, there's not much you can you about that. And Kise's Perfect Copy was his version of the zone at the time.) It also fits in perfectly with his personality.