Marks must maintain a consistent trajectory
There are a lot of situations where you want to capture something that has a repeating pattern. When doing so, don't attempt to capture it all at once. Instead of zigzagging back and forth and repeating the same pattern, break it up into separate strokes, each one with a single purposeful trajectory.
When dealing with a smoother wave, it's often better to start with a smooth line, and then build individual "bumps" onto it. We'll use this quite a bit in Lesson 3, when we draw the wavy edge detail on leaves.
When drawing something more jagged, like fur, it's important that you purposely design the tufts of fur, making a stroke to go up, a stroke to come back down again, another going up once more, and so on. This allows us to be intentional in how we make the marks.
Zigzagging back and forth, on the other hand, will cause us to go into auto-pilot, as all repeating patterns tend to. This will result in the same, boring, flat outcome that will lose all sense of character and intentional control. Avoid it wherever you can.