A summary of Baal HaSulam’s article “There Is a Difference Between Corporeality and Spirituality” (article #164 from Shamati):
In corporeality, the force precedes the act. Such is the arrangement at the end of correction: no act is done until there’s enough strength to carry it out.
In spirituality, the act precedes the force. Everything takes place according to a step-by-step disclosure of desires suitable for correction, from the easiest to the most difficult. Therefore, the work needs to begin even before one has the strength to carry it out.