Owing to his lack of formal training in martial arts (particularly the 1 Yeung Finger Technique considered prerequisite for training in the 6 Mak Divine Swords), Deun Yu's 6 Mak Divine Swords was very unreliable. Sometimes, it'd work, but just as often, it would short circuit on him.
Clinically speaking, what was the problem? It couldn't just be because he didn't know what he was doing because he could get it going some of the time. What was he usually doing wrong that made the technique not work? Later on, when he got it to work more reliably, what did he figure out to get it that way?