I think I know the hitbox, it is like everyone says it needs to hit head or feet to work. well actually I just have some evidence that proves what we already has said.
I went to the japanese DOJO went to the part where it tells about links dtilt, I then translated the text
into english with google translate(the swedish translation was even more stupid than this):
Under the strong links to the attacks, the drop in part under attack.
which I further want to translate to this:
Links Strong down attack, makes meteor smashes in the part below the attack
and into smash talk:
Links Dtilt has a spike hitbox underneth the blade.
then if you look at the image you see that the hit animation is underneth the blade(the white shiny-ish star shaped thingy)
conclusion: the spike hitbox is outside the regular one, when both hitboxes hit the regular outpriories the spike hitbox. Go for head and feet. im going to test this at monday.