Slides in FED

[LMan]

can someone untangle this for me:

FED 2-32-1-2(f) " A slide is illegal if the runner, on a force play, does not slide on the ground and in a direct line between the two bases."

Sounds like he MUST slide here, and of course, the slide must be legal.

but elsewhere, FED states a runner is NEVER forced to slide, but if he does, the slide must be legal (8-4-2(b) note).

Since I cant see offhand how a slide NOT on the ground can be legal (or is even a slide at all), how can 2-32-1-2 (f) be interpreted to NOT require a slide (in contradiction to 8-4-2(b))? Isnt that what the FPSR is all about?