In Knights in the Nightmare, High Skills can be used under the following conditions: The weapon selected must be capable of performing a High Skill, and the knight's elemental alignment must match that of the weapon's. The elemental condition can be bypassed if certain Effects are active (namely, 78 for male characters, 79 in Law Phase, 81 in Chaos Phase and 87 for females.)
When the charge gauge reaches MAX, another Charging tier will begin to fill; it must be fully charged for the High Skill to be unleashed. After it is performed, however, the weapon cannot be used again that turn. Also keep in mind that the use of a High Skill consumes one full point of VIT from the user.