It seems like every theory program has a different name for this
Ic construction. I admit I have never seen it called that, though I have seen a variety of other names, including
V6/4, and just plain not labeling the chord.
Nevertheless, the idea is the same. The chord labeled
Ic is not a functional chord. Rather, it is a double suspension over the bass, and its resolution in common practice music needs to follow specific rules. Note that the intervals above the bass are (octave displacements of) a sixth and a fourth above the bass. The pitches happen to be the same pitches as the tonic (
I) chord would have, but the chord cannot function as tonic. It must resolve to the dominant (
V) chord with the suspended pitches going downward by step. That is, the sixth above the bass must go down to the fifth above the bass in the same voice, and the fourth above the bass must go down to the third above the bass in the same voice.
Because of the rules about downward resolution, the sixth and fourth above the bass may not be doubled in a four-part texture. To do otherwise would require parallel octaves in the resolution, which is (of course) forbidden in this style. Therefore, the bass must be doubled in the
I have never heard nor seen the rule about the octave leap in the bass. If the
Ic designation implies something beyond the designations with which I am more familiar, then perhaps it is a rule in that case, but I have seen many, many instances (at least in textbook examples) in which the bass simply stayed on the same pitch. I can imagine that such a rule would create difficulty in certain circumstances. Of course, the octave leap is permissible, provided that no other rules are broken.