Here's a bit on it I wrote from my guide, hope it's somewhat helpful:
VI.)HARD KNUCKLE
Down Aerial- A pretty unique move here, a meteor smash from a decent distance away. Despite working best at a range though, it does in fact do more damage up close, which is worth noting. Now, I feel the best way to pull this off is to:
1.Jump off, making sure to keep a jump
2. Align yourself with the opponent
3.Using your double jump, proceed to dair them as the first jump was bait.
Please include mix ups, mix ups are an important part to any healthy metagame.
Now, this move
can be used to combo, although it
can be teched. To use this onstage with no lag, jump, gain a bit of height, then jump again as you use it. Depending on when you jumped, this can be fast falled towards the end of its duration, although it's safer not to unless you've got the distance down. This will allow you to utilize it, say, after a crash bomb goes off, and depending on whether or not they tech ti can be followed with an Up Tilt.
Crash Bomb ->Dair->Utilt https://www.youtube.com/watch?v=N2OrJiY5tfM
Overall, given its unusual nature, I'd say it's about average. It has a small sweet spot, and is quite laggy, but when properly spaced its safe and can be used to at the very least follow up a move or two. I'd have scored this as below average, but the range on it is what somewhat redeems it.