Delusions allow a character to do things outside the normal bounds of Aspect invocation. While this can be extremely useful and powerful in the correct situations, it requires that a portion of ones ego to be subsumed by its existence. Frequently this trade-off is very beneficial, and can be performed in a number of ways, but all involving magic usage of some description.
The Delusion has two components, an Aspect, and a need. Like regular Aspects, this one provides automatic bonuses to appropriate Challenges, but it cannot be invoked to add extra bonuses, or reroll a Challenge. However, the Aspect can be invoked to perform an appropriate action that wouldn’t normally be possible without the delusion.
For example, a 'Cyberarm' Delusion would provide +1/+2 to regular arm-related things. When the character with the Delusion wants to punch through a brick wall, or jam a bulkhead door shut, spending a point of Essence makes this possible. To refill the Delusion’s capacity, the character simply must fulfill its need; in this case, eating and digesting a high-energy protein bar.
To purchase a Delusion, a point of the characters Essence capacity must be dedicated to it. This reduces the total amount of Essence that the character can store, but provides the Delusion with its own capacity that is typically refilled in an easier, quicker, safer, and/or more accessible manner.