[807] Therein, what is skill in gain (āyakosalla)? "When one attends to these phenomena (dhammā), unwholesome phenomena that have not arisen do not arise, and unwholesome phenomena that have arisen are abandoned; or else, when one attends to these phenomena, wholesome phenomena that have not arisen arise, and wholesome phenomena that have arisen lead to increase, to abundance, to development, to fulfillment" — the wisdom (paññā) in that regard, the understanding, … non-delusion, investigation of dhammas (dhammavicaya), right view: this is called skill in gain.
Therein, what is skill in loss (apāyakosalla)? "When one attends to these phenomena, wholesome phenomena that have not arisen do not arise, and wholesome phenomena that have arisen cease; or else, when one attends to these phenomena, unwholesome phenomena that have not arisen arise, and unwholesome phenomena that have arisen lead to increase, to abundance" — the wisdom in that regard, the understanding, … non-delusion, investigation of dhammas, right view: this is called skill in loss.
All wisdom whatsoever that is the means therein is skill in means (upāyakosalla).
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.