Aren't they the same thing? There's a particular reason to do Shiko, as opposed to say Fune Kogi Undo, but for all practical purposes they're just facets of the same jewel. But I'd be interested in either hearing discussion about why Shiko particularly as an exercise or why winding in Shiko. Maybe Dan can chip in, since at the beginning of the post he was telling Chris:
Whatever you do ignore everything in that Sumo link Chris provided. It's nothing a martial artist wants to do. This is NOT the Shiko you have read us talking about nor want you want to be doing.
What exactly is a martial artist doing in Shiko that is so helpful? Why is winding/spiralling important? And so on. The discussions about the how and why would probably be helpful to everyone.
It's probably because it helps to develop that whole body connection (from head to finger to toe). This helps a martial artist because it makes them feel stronger to their opponent.
Buddha's Warrior Attendant move in the Chen Taiji form is pretty much shiko. I rotate my dantien (my torso) to the right to pick my right leg up and rotate my torso left to drop the leg back down.
It's better than using just your hip flexor muscles to just pick up that leg. That's just training your hip flexor muscles. I guess that's okay if you want nice looking hip flexors.
But if you use the winding connection in your body (also connected to the ground), the leg should feel stronger to the opponent when the leg is raised--provided your body's connection is conditioned well. It's because you have that whole body behind the leg being raised. That can be good if you want to knee someone, walk, or do whatever technique that involves you picking your leg up.
These are just my observations.