The fourth line is the third line done correctly. Legge's translation gives good fortune to firm correctness, says the occasion for repentance disappears, shows the fence opened without the horns becoming entangled, and compares the strength at work to that of a large waggon. Every element answers the line below it. The same obstacle is met, the same power is applied, and this time the barrier gives way while the subject remains free to move afterwards.
Structure explains the difference. This stroke is strong in an even place, which the ordinary rule counts as incorrect, and it stands at the base of the upper trigram Zhen, the first line of thunder and the fourth firm line in a row. It is the head of the accumulated force of the entire lower half. What it does not have is the correctness of position the third line had, and the hexagram treats that absence as an advantage: a line with no formal entitlement to charge is the one that thinks about how.
The waggon carries the point of the comparison. A ram is all impact and no carriage, so its strength ends at the moment of contact, while a waggon moves its power along an axle and keeps rolling once the way is clear. Great Power is a figure about force that overreaches, and this line is its one unambiguous demonstration that force need not, provided what stands behind it was built to keep going.