Energy is deducted after the conditions and hexes are removed and cannot subtract energy below zero. This makes it useful to remove condition and/or hex stacks, as well as to combine with focus swapping.
Works well in a signet build as part of team condition management.
Heroes seem to only use this skill when multiple conditions are on an ally.
If the goal is to remove single hexes using a signet, consider Hex Eater Signet, a MesmerInspiration Magic signet which gives energy instead of costing energy.