Timing in a picture

I found this difficult,  we were told to draw the model and then she would move to a slightly different position. She changed her head direction and shifted her body slightly. We had to draw over our original drawing, this is what I found difficult. I couldn't tell which line was either the one I had just drawn or the one from the previous position. She then changed back to her original position and we were told to finish the original drawing off which confused me further. I found it difficult but I saw how the body shifts and changed with the slightest movement and how you can get a sense of timing in the picture.