I want to prove
(Rolfsen) If an embedding $e: S^1 \rightarrow \mathbb{R}^3$ has only one relative maximum and minimum in the $z-$direction, then $e(S^1)$ is unknotted.
In particular, I do not have a clear intuition why the condition of relative maximum and minimum allows for the knot to be equivalent to the disc. I would assume that a "stretched out" trefoil knot (along the $z$-axis) would suffice as a contradiction to the aforementioned statement?
Even if I could get past the intuition, I am not sure how I could sufficiently construct a way to show isotopic equivalence (in general).
Any help would be much appreciated.
Edit: Something like this, where there is a relative max and min?