Advancing with difficulty ; the ox's nose is pierced.
But this fiery nature is hard to control.
Dragged here and there, you stray through cloud-covered forests.
"Advancing with difficulty" means that you have to endure physical hardships without caring whether you live or die. It is at such a time that "the ox's nose is pierced." Yet even though its nose is pierced, its "fiery nature is hard to control." Whatever you do, the ox will always try to retreat quickly and run away. At times, though, the practice makes good progress, like that of a boat being pushed over ice. But after a while it ceases to be so easy and becomes as difficult as trying to force a horse to drink water when it doesn't want to. No matter how close you succeed in bringing the horse to water, it will keep avoiding it and running off. To control immediately the "fiery nature" of the ox is indeed difficult.
"Dragged here and there, you stray through cloud-covered forestes." In such a place the moon only shines sporadically. After you have caught the ox and pulled it toward youself with a great effort for a while, it will suddenly pull you off in another direction. You try to pull it back, but again it manages to drag you elsewhere. It goes on and on like this. Now you are struggling with great difficulty in a cloud-covered forest. The clouds are dense and the forest is thick. And you keep on straying here and there, catching hold of the ox and trying to pull it in your direction.
What exactly is this difficult time ? It refers to the stage when the meditation is composed partly of the hwadu, partly of distracted thoughts, and partly of sinking into dullness. At this time, these three factors seem to be competing with one another : at some times you find yourself in a state of dullness, at other times beset with distracting thughts, and at other times concentrating on the hwadu. This is a vey difficult period because now you are really fighting with the ox.