Think leading position and attention + presentation.
The pony must negotiate his way through the S-bend without stepping out of the poles. To present your pony to the obstacle, walk up to it, keeping his attention, halt in front of it, turn to face him, and proceed through the S-bend, walking backwards yourself, watching your ponies feet. At novice level you may step outside of the poles without loosing points. Teach your pony to use all the space available to make the turns effectively.