I will have a go and this is just an opinion.

What you are starting with is a generic sausage shape with a centre line. What you are doing with the ellipses is pushing your interpretation of how that sausage is in 3d on to it. This will depend on whether you see it as receding, advancing or changing direction from your perspective. The same sausage could be interpreted different ways. So it's not a logic exercise, you are choosing how you wish it to look and to give it a convincing 3d form. I tend to stare at the form to see if I can intuit a certain 3d shape from it. If I stare long enough a shape always starts to suggest itself.

Hope this helps.