Given that the indirect WH-feature checking movement is covert in English, the prediction is that an XP can be pied-piped even when the WH-phrase is not in its Spec overtly as long as it could move into the Spec covertly.
in 14, we can say that the WH-complement of P moves 'covertly' to the Spec of PP and percolates its WH-feature to the PP .4 of this 'covert' movement and feature percolation, the PP to whom undergoes 'overt' pied-piping WH-movement to the Spec of CP.