Six in the second place means

Keeping his calves still.

He cannot rescue him whom he follows.

His heart is not glad.

The leg cannot move independently; it depends on the movement of the body. If a leg is suddenly stopped while the whole body is in vigorous motion, the continuing body movement will make one fall.

The same is true of a man who serves a master stronger than himself. He is swept along, and even though he may himself halt on the path of wrongdoing, he can no longer check the other in his powerful movement. Where the master presses forward, the servant, no matter how good his intentions, cannot save him.