In the commonly reported Muʿtazilī account, capacity exists before an act, allowing a person to choose between acting and refraining. For that account, pre-existing power is important to explaining responsibility and obligation.
The position defended here instead locates the specific power of an act alongside that act. This does not imply that a human being is wholly helpless in every respect before acting.
One source of the disagreement is terminological: one account calls the previously available ability to act 'capacity,' while the other reserves the relevant technical term for power connected to the actual occurrence.