As you sketch, a variety of dots appear around your line. These dots — called "pop points" — are useful tools you can use as leverage to join your lines at common junctures and align them with other parts of your sketch. There are two types of pop points that appear as you draw your sketch:
When a pop point appears on screen that you would like to use, simply click it or press the appropriate directional arrow to draw a line connecting the end point of your current position to the location of the pop point. You can also move your cursor near a pop point and press the letter J on your keyboard to jump to the nearest point.