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An energy spike in general refers to gaining or losing a large amount of energy in a short amount of time. This is often useful offensively, both against enemy healers to cause significant unexpected energy problems, or on yourself to power rapid casting of expensive skills.
Power Leak is one example of an energy spike. Interrupting 15 energy spells will cause the target to instantly lose ~30 energy, depending on what kind of energy management they have in place.
Auspicious Incantation is a popular skill for gaining large amounts of energy quickly. When used on a 15 or even 25-energy spell, the skill can easily fill up your energy bar, allowing for repeated casts of spells like Rodgort's Invocation, Lightning Hammer, and Panic.