Back stroke means that when writing a stroke, when the pen is ready to leave the paper, the nib returns to the original stroke, and the final stroke is generally round, such as horizontal and vertical.
After writing, the front end of the pen does not return strokes, and the end of the strokes is pointed. For example, most strokes are left, pressed, ticked and picked.
You can see the difference between hanging needle vertical and hanging vertical. One is the exposed front and the other is the back front.