...Some degrees are more stable than others (1, 3 and 5; 1 being the most stable) and some degrees are more unstable (2, 4, 6 and 7; 7 being the most unstable).
The unstable tones are called tendency tones.
...How does that [solfege/tendency tones] work if there is an underlying harmony?
I think the way to think about this is: solfege/tendency tones and functional harmony are just "two sides of the same coin." Solfege tones don't happen over a harmonic background, the solfege tones are the harmony! I don't mean to be glib. We don't fit solfege to pre-existing functional harmony. It's the other way around. When tones move according to characteristic patterns of solfege/tendency then functional chord identities emerge.
The easiest way to think about it is to consider the tendency tones ^2, ^4, and ^7. Together they make a viio6 chord which has a strong resolution to the tonic chord.
The next question that comes up is: "how to consider the dominant tone ^5?" The dominant can be a chord tone in both the tonic I and the dominant seventh V7. The V7 chord has a strong resolution to the tonic I. This seems to case ^5 in an ambiguous role of both stable and unstable. Let's try to clarify this.
Tonic ^1 and dominant ^5 are both stable degrees. The are the pillars of stability in tonal music! When a plain triad is built on ^5 the V is a stable chord! This stability can be clearly head in the half cadence. The leading tone ^7 is in the V chord so it does resolve to I but V should be considered stable as the music can cadence on it.
When the seventh is added to V to make V7 the chord becomes unstable. FA has the tendency to move to MI and that is the source of the unstability. My understanding (mostly from William Caplin's writing) is that the unstable V7 would not be the goal of a half cadence.
So, the level stability of dominant harmony is the interplay of stability from ^5 and the instability of ^2, ^7, and ^4. Especially important is which tone is the bass! While V is stable, V6 is not.
A similar interplay of tone stability happens with the IV chord and I. Obviously the ^1 degree in both chords is stable, but the ^6 and ^4 in the IV chord are unstable and resolve down to ^5 and ^3 respectively. It is interesting to examine bass tones and inversions with these chords. With IV64 to I the inverted subdominant is obviously the unstable chord. With that specific voice leading we clearly hear LA and FA resolve down to SOL and MI respectively. But, what if we use difference inversions and voice leading? With I6 to IV the tonic chord is unstable and IV becomes relatively stable. This movement can be interpreted as a tonicization of IV in which case the movement of MI to FA becomes TI to DO.
However, if we don't have a tonicization and instead it's a move to a half cadence with I6 IV V we get to another important concept: tendency tones do not always move according to tendency! Again, the harmonic context is critical. In this half cadence progression FA is the stable root of IV. Compare this to another harmonic context: V42 to I6 where FA is in the bass, but as a chord tone it is the dissonant seventh of the dominant chord.
The examples above may be confusing, because the context keeps changing: tonicization changes the tonic, inversion makes stable chords unstable. But that gives us the background information for a generalization: tendency tones exhibit their tendencies when they are used in dominant harmony. In other words V7 to I is where we see tendency tones in action.
This leave LA as the tone with probably the least tendency, or perhaps the most ambiguity. LA's tendency to move to SOL is best understood by IV64 to I a contrapuntal movement to resolve the unstable 64 chord, or in cases like ii6 to V and IV6 to V the move LA to SOL is about doubling the V root instead of the chord 3rd/leading tone. The falling thirds sequence is a case where LA doesn't follow the tendency to SOL - I V6 vi iii6 - in this case LA moves to FA notice how that non-tendency movement doesn't involve V nor I.
- tendency tones are most clearly observed in dominant harmony
- tonicization redefines solfege members and effects tendency
- chord inversion and counterpoint effect tendency
- tendency tones do not always move according to the baic definition