Elementary, my dear Watson. There is a rook to the left of the knight (and out of view in this picture) that pins the equestrian piece against its monarch.
It`s interesting, though, that under the more recommended images there is not a single one showing plain Staunton chessmen on a "normal" board. They are all very "special", probably because people see no use in a good, but "representative" shot.
I only have very unique chessmen, but if one of you owns the Staunton ones, with a nice wooden board, just make a perfect shot showing the starting position in a good light and submit it from the beginning as "representative shot". I`d recommend it first!
