I see it as a lower body exercise. But it works probably more muscles than anything else you can do.
The core muscles - which as a group include glutes, hamstrings, obliques, RA, Erector Spinae, TA, QL, hip flexors etc... - are heavily worked.
If you look at the core muscles, they are pretty well designed, and I think its good if people understand them and what they do.
Iain - Do you mean; the internal obliques, external obliques and lats run diagonally and sort of go "around" the midsection to opposite sides, to create a 'corset' effect?