The fifth line holds the ruling place of Nourishment, and it holds it as a yielding stroke. Legge's translation describes a subject who departs from the ordinary course, abides in firmness, and finds good fortune, while adding that the great stream should not be crossed. Those two halves are one judgement rather than two. The line can nourish and it cannot venture, and what secures its good fortune is accepting both facts rather than acting on only the first of them.
By the ordinary rule a yielding line in this odd place is incorrect, and the departure from the regular course that the text acknowledges is exactly that: the ruler of a hexagram about feeding others is not strong enough to feed them from his own resources. What makes the position workable is the strong sixth line immediately above, which the hexagram treats as the origin of nourishment. The fifth line's role is to hold steady and stay in correspondence with that source, distributing rather than generating, and the insistence on firmness is the whole of what is asked.
The great stream is the classical image for a large undertaking with real risk and no way back once begun. Withholding it here is not timidity but accurate self assessment. A ruler who provides for others by drawing on a source above him can do that indefinitely, and cannot lead an expedition, and the good fortune the line promises depends on knowing which of the two he is being asked for.