The approach to C5 and C6 in measure 49-50 seems like an augmented-sixth chord. An augmented sixth between db and bnatural resolves in half steps to C, suggesting an augmented sixth chord in the key of F. However, this measure has no sign of scale degree 1, which is essential to the composition of an augmented-sixth chord. The dflat in measure 49 alludes to the augmented-sixth chord used previously with this chromatic melody. I expect Beethoven chose not to include a complete augmented sixth chord this second time because he was headed toward a cadence of IV-V-I, and an augmented sixth chord build off of dflat and bnatural would have shone a huge light IV, screwing up the forward motion towards V7 and I. ((((edit- I think Rachel is right and that this is a V65/IV with a b natural passing tone, but all I say about alluding to the earlier melody and such still stands)
The chord structure of m44-m47 and m48-51 is similar:
In the first phrase, IV6 is used to accommodate smooth voicing toward Fr6, which draws attention to the V chord by pulling tendency tones toward scale degree five. Scale degree two is shared by Fr6 and V, but sounds exotic against the eflat in Fr6.
The melody in the second phrase begins identically, only an octave higher. This alone brings more attention and intensity. The chromatic movement from m.46 is borrowed, but put above the I6 in m. 49, which creates more urgency than before, because upward chromatic movement didn’t take place until the third measure in the first of the two phrases. Once IV is reached in measure 50 there are rests separating IV, V7, and I, drawing attention to this cadence, but this is soon interrupted by tumbling triplet figures which propel further into the piece.
As a whole, this movement feels urgent and propels forward like a wheel. Sections such as m. 43-51 feel like a break from all of the commotion and create a sense of lifting up. Through much of this piece I can envision myself in a rush, stuck somewhere between an urgent scramble, and a brisk but leisurely walk. Occasionally there is time to look side to side and appreciate what I am passing- particularly this section. It is as if something divinely simple has caught my eye, such as a flower, and I stop to appreciate it for a few moments, and then during the cadence in measure 50 I suddenly realize my destination and that I am late, and jolt forward. I think it is important to be aware of how the feeling of tempo and urgency rises and falls, but the performer doesn’t need to do anything more than follow the text to make this happen, because Beethoven has written it in the notes.
The performer must not get in the mindset of “longer notes! I should stretch this out!” Beethoven has done that already by morphing triplets into eighths, eighthnotes into quarter notes, and quarter notes into half notes. The tempo should be maintained exactly to ensure the listener that the piece continues to move forward, and that motion (the driving force in this piece) still exists, even during restful, uplifting moments.