Hexwraiths have fly, so you're allowed to move over other units, so long as you don't end your move within three inches of an enemy unit. In order to 'move through' an enemy unit, your movement must bring the model over the enemy unit at some point.
For the most part, though, it isn't an ability worth using, as you'll rarely be positioned just right that you can move over an enemy unit while still ending your move three inches away from them, and you're really better off just fighting anyway. If for some reason you need to break off from a fight, though, remember you can still use this ability while withdrawing, so... that's something I guess?
But yeah, 5 hexwraiths average less than a single wound from this ability. It's cute, but it's not terribly relevant.
Note that, as of the newly official FAQs, movement is measured point A to point B, not over the actual path followed, which means hexwraiths are no longer allowed to move partially over an enemy then return to their previous location, they now have to travel completely over the enemy and land more than 3" behind them.
As such, the ability is that much less likely to ever be used. Honestly, you should probably just ignore it outright, the unit doesn't need it to function, and its so hard to use, and does so little damage should you ever pull it off, that it really isn't worth the hassle of caring about it at all.