I first encountered this a while back, and though I was able to reproduce it sometimes, I couldn't do so reliably. Today I sat down and tested, and discovered that it's actually quite simple. Ike's sword has a hitbox while he charges Eruption, but only for a few frames, and only if Ike is in midair. (That's why I had so much trouble reproducing it: I tested on short characters like Kirby, set to "stand," and Ike holds his sword way too high to hit them unless they jump.) The hitbox seems to be from f-smash. It deals 22%, just like an uncharged f-smash, and it has similar knockback. Plus, there's a windbox.