Nobody has posted it here, but I saw it on the Nexon forums about a month ago.
http://vindictus.nexon.net/Community/#%2Fshowthread.php%3F270644-Fionas-behold.%26nxid%3D15
It's not a very popular tactic here, but it's an interesting tactic.
The theory for those who are curious is that the animation when you exit a sprint (the slide fowards), it shifts your hitbox fowards by a noticeable amount. Ingkells has fairly small and precise hitboxes, so he's clearly the best example for this. People think that the animation throws Fiona's hitbox so far fowards that it places her hitbox inside Ingkell's body, so all of his attacks miss.
Lann should be able to do this in theory as Fiona and Lann share sprint animations (to my knowledge). Not sure about Evie or Karok.
Really it's impractical apart from Glas and Ingkells, since most attacks last too long. Also you're better off rolling/sprint smashing out of the way of Ingkells as that allows you to get a smash ready. That or if you're trying to do Shieldless solos.