Not only Mozart's musical phrases, but every musical phrase can swing. We do not talk about rhythm. We talk about phrasing and articulation. These parameters of course are also influenced by rhythmical structures and allover textures of the musical scene, we're navigating in at this special moment of musical time.
But let's have a look to a rather simply structured passage after the main-theme in Mozart's famous C-Major-Sonata KV 545.
(see picture below)
We find a simple up-and-down movement in semiquavers, accompanied by harmonic chord-like intervals, which should also mean a certain rhythmic accent on the one and the four of the bar. This
texture, when performed in up-tempo, seems to be originally taken from (in this period of history) typically used cembalo accompaniment (stroke)-techniques and modified into something like a
rhythmical on-off-time machine, as played about 150 years later by jazz pianists to support the rhythm-flow of their right-hand-phrases with on-and-off accents of harmonic chord-accompaniment.
Unfortunately this chord-accompaniment by Mozart is normally played like a little embarassing but unavoidable side-issue below the actual main issue of the major-scale melody. But…the
scale-movement itself is trivial without its rhythmical counterpart-accents, which should make the whole thing not just move but even swing. Of course, Mozart's chords are not off-accents like in
jazz-piano-accompaniment. But nevertheless these chords are the impulse-motor for the semiquavers in the melody-flow.If they are underrated, the flowing semiquavers miss an undispensable
harmonic-rhythmic basement for their own running tempo.
This is only one crucial point of a deeper analysis of Mozart's "swinging Allegro".
The other point refers to the articulation of these semiquavers. Again, normally these notes are played in a kind of even and unpretentious manner, avoiding any accents and tempo shifts: Interpreters aim at a virtuosic flow of glittering equivalents, as played to bring the form's narrative from point A to B, but without any bigger marks of expression or meaning. It is only a transient part and therefore it has to sound simple and smooth. But may be a little boring, too?
We believe that there should be a distinction of articulation between the up- and down-movements of the semiquavers. The upwards-scale can be played in a strong legato with a slight natural crescendo whereas the down-movement would follow in a kind of slowing-down portato-like phrasing. No rubato playing of course but an intended impression of different sound textures for the same scale movement. This may be necessary to provide the phrasing with musical tension and to give expression to a texture which could be missunderstood as “filler words“ just to get to the next important theme of the musical form.