I am trying to understand the following description:
Diminished Chords Functioning as Dominants
"Diminished 7th chords may function as rootless 7b9 chords. The diminished 7th chord occurs naturally on the 7th step of harmonic minor tonality with the label viio7. The same label is used when diminished chords function as secondary dominants.
An important difference in the analysis of a diminished 7th chord is that in order to be considered a functioning secondary dominant, it must be followed by one of it's intended targets: a diatonic chord half a step above any of it's tones (any tone of a diminished 7th chord may be thought of as it's root).
Motion of Diminished chords with secondary dominant function in C major:
viio7/ii (C#o7) target is: iim7 (Dm7)."
The text goes on to list the four other V - I resolutions in that key. I do not understand how they have arrived at C# but must be something to do with the first sentence of that text. can anyone explain this? I am a beginner at piano Jazz and do a lot of theory study to go with it so go easy if you can. Thanks.