Focus , actually. Not only does it crumple a grounded opponent (airborne opponents simply get launched) on a successful counter-hit or when slightly charged, it gives one hit of super armor so you can't be easily hit out of it. I doubt it would work the same way as most counters in smash do, I think it would work as a armored, cancellable(!) attack. One that leads to very damaging followups if it hits. Very useful indeed.