Speaking of the spike, I've still been hearing two answers, the various leg parts and the hands.
I'm almost positive it's the hands myself, because it always seems to work that way for me.
The reasoning is luigi is like one of these \ when he does his dair, so the back hand is more likely to hit than the front hand because his legs aren't sticking out forwards, and I've seen luigi's spike work from the front.
I think the ONLY time you should attempt spike is on snake's recovery, because it's easy to line up. Other than that, just go for bairs or sweet dairs to edgeguard.