Spirit Reservation
Persistent Buff Skill Gems reserve an amount of Spirit based on the Gem, such as 

Herald of Ash
 which reserves 30 Spirit. Importantly, Persistent Buffs granted by items such as the Malice Skill shown above, do not require a Spirit reservation. Note: Support Gems with a Cost Multiplier do not increase Spirit reservation.

How to Activate a Persistent Buff
Unlike in Path of Exile 1, persistent abilities are not enabled via the Skill UI at the bottom right of the screen. Instead, you'll need to activate the Persistent Buff by opening the Skill Panel (default: G) and right-clicking the icon above the socketed Persistent Buff Gem. Persistent Buffs granted by items are automatically active. Note: you must have enough unreserved Spirit to be able to activate it.

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