I'm playing through TP right now, and Shin, you were right about the Spin Attack being instantanious. At the moment of input, anyone within range takes a hit, regardless of whether they are behind you, in front or wherever. Also, the "shockwave" from the move extends the attack's duration so that even though Link has finished the attack, an enemy will still be struck if it enters his attack's field of influence. This means that the Spin Attack has virtually no startup or cooldown. One more thing; the shockwave from the attack reaches above and below the sword's path of travel a great deal. A trait like that would nullify aerial strikes as well as attacks from below.
To whoever it was that said that the Spin Attack was weak and useless; I'm calling you an idiot. 👆