So far three possible labels have come up:
- passing chord
- tritone substitution
- "planing" or "parallel harmony"
All the above may be appropriate depending on the harmonic context.
I think you want to consider the harmonic rhythm which is the rate at which proper chords change. In many, many songs the harmonic rhythm is usually one or two chords per bar. When there is motion happening much faster than the harmonic rhythm, it often makes sense to regard it as embellishment, some kind of passing or auxiliary motion, even if it incidentally forms "chords."
The other thing to consider is functional harmony, basically your flow of subdominant, dominant, tonic chord progressions. A lot of harmonic passages can be viewed as various elaborations of otherwise simple, functional progressions. A fairly common thing is to start on a tonic chord, move voices by little half step changes through a series of dense, chromatic seventh chord harmonies, which eventually arrive on a dominant chord to end the phrase. You can label all the chords, but don't overlook the importance of the phrase being and elaboration of
I ... V.
A series of tritone substitutions can definitely create a series of parallel descending chords. If you're being picky, those chords should be some kind of dominant. But, as Richard points out in his answer, some jazz folks use tritone substitution to "explain" chromatic progressions, even when the chords are not functioning as dominants.
Ruby My Dear by Thelonious Monk provides a nice example combining functional harmony and a parallel passage - although it uses an ascending motion, not descending. The first two phrases are just
ii V I sequenced up a whole step, but between those two phrases, while the melody holds a whole note, there is a lead-in of parallel minor seventh chords. That parallel passage is just embellishing the main chord structure, it's a type of passing motion.
In the examples you linked, if I'm hearing the right moments, they happen fast, and feel like they are between the main chords. That's passing motion.