Activity is a part of the Hibernal Aspect of the Marrow Fysa.
Passive magic is switched to an active magic-- that is, passive or uncontrollable effects can now be triggered on will. This applies to ALL magics on the user.
These now-active magics have greater strength and flexibility, but also a greater cost of energy and can be debilitating to use frequently or without Esse.
Leyr can use this to transform on will rather than on emotion, for example, but it is more taxing.