大般涅槃經義記 4.179

4.179

第三法力得果之中，義別有二：一明如來外化行果、二「如蛇」下辯明如來自利行果。前明外化，即是答上生死大海中云何作船師；今明如來得涅槃船，周旋往返，濟度眾生，故能於彼生死海中得作船師。

In the third section, attaining the fruit through the power of the Dharma, the meaning has two parts: first, explaining the fruit of the Tathāgata's practice of transforming others externally; second, from "like a snake" below, explaining the fruit of the Tathāgata's practice of self-benefit. The former explains external transformation, which is answering the question above about how to be a ferryman in the great ocean of birth and death; here it explains that the Tathāgata attains the boat of nirvāṇa, circling and shuttling back and forth to cross over beings, and thus can serve as a ferryman in that ocean of birth and death.