Turn around lightly with heavy hooves, and work hard all day without showing your face.
The first sentence is the "physical characteristics" of the donkey, and a (white) line extends from the back to the tip of the tail.
The last sentence has two meanings. One is that people feel light, so it is worthwhile to give some discounts on food.
The other is that, in the second sentence, it was covered and really didn't show up.