The fifth line is the strangest sentence in the hexagram and it neither promises nor threatens. Legge's translation describes one with a chronic complaint who lives on without dying. There is no good fortune here and no misfortune either, only continuation under a condition that does not lift. In a hexagram about energy and its movement, the ruling seat is occupied by something that cannot move freely and cannot be cured, and the text declines to dress that up.
A yielding stroke holds this odd position, incorrectly placed by the letter of the rule, at the centre of the upper trigram Zhen. The seat of rule and the source of the hexagram's energy have come apart: authority sits here, while the single strong line immediately below at the fourth place supplies all the actual movement. The classical reading of the chronic complaint is precisely that constraint, a ruler pressed by a minister too strong to be dismissed and too necessary to be lost.
What the line contributes to Enthusiasm is a corrective the hexagram badly needs. Every other position is concerned with generating, borrowing, chasing or losing momentum. This one describes what it is like to hold a place while the momentum belongs to someone else, and it makes the honest observation that such an arrangement is survivable. Not thriving, not resolving, but continuing, which the text presents as an outcome worth naming in its own right.