A mystical sword technique involving a hypnotic affect caused by a certain pitch sound similar to whistling. It forces the opponent to strike in a stabbing motion stepping toward Glen, where by he times the technique to strike just before they do. The whole point of the Lorelei technique was to cut into the foe a split second before the opponent could strike. A split second. If for any reason his timing was thrown off by even a fraction, he could wind up on the receiving end of the very same stroke from his opponent. With basically no time to spare, the attack literally involved the swordsman risking his own life. But that was precisely why it'd been worth while for this fearless seeker of knowledge to sweat blood to master this technique.
The technique may allow a burst of speed and agility, this is not necessarily described in the novel, but is depicted in the manga.
The power of this technique is enhanced many times by Glen being turned.