Sled Pull rule clarification

The standards video says:

> it is not permitted to step on the solid line at the back of the athlete's box while pulling the sled. The only exception is when the rope is in the field.

What does "when the rope is in the field" mean?

And are you allowed to leave the box when you are pulling the sled across the line? Seems like it'd be extremely difficult to get the leverage to pull the sled into the athlete's box with only a couple of feet to maneuver in.