A newbie question regarding sweeps:

The attached sldprt (2013) is a variation of one found elsewhere in the forums. What I am stuck on is precisely how Sketch3 was created from the sketches and features preceding it. I understand the creation of the Surface Sweep, but getting from there to Sketch3 is eluding me. My various feeble attempts to use Convert Entities fail because the sinoidal loop does not appear in the graphics window during the specification of Convert Entities properties. So apparently I am starting out at the wrong place or taking the wrong steps to get from the Surface Sweep/Surface Body to the 2d Sketch3.

In the end, what I'll do i with this is to incorporate use of the sinoidal loop in another part, to create an Extruded Cut from an Extruded Boss. It will be a rather more elaborate version of the depiction below. Fortunately, once I understand how to get from the surface sweep to the Sketch3, I'm pretty well home free, in that creating the extruded cut from Sketch3 is trivial.

Thanks for taking the time to look at this. And, given my newbieness, please note that a generality such as "Just Convert Entities" does not give me detail sufficient to complete the task. I am only gradually filling in the swiss cheese of my Solidworks tool knowledge.