Where does
the thread go next?
One space on the front. One space on the back.
Choose a face to build the line.
The blue band is the cloth. The side view opens it up so you can see the hidden thread.
Start with one interval on the front.
Why does the thread disappear?
A gap in the front pattern is not a gap in the thread. At each marked point, the needle changes faces. If you start on the front, intervals 1, 3, 5 and 7 are visible there; intervals 2, 4, 6 and 8 are on the reverse. The side-view drawing exaggerates the space around the cloth to show that path.
Each interval here is equal. This introduces a simple grid-based running stitch, not a rule for the proportions of every sashiko style.