One of the etudes I am working on right now is Duport #6. I believe that is one of 2 or 3 etudes in his (Jean-Louis's) 21 exercises at the end of the Essai that are not written by him, and as I recall this one was written by his older brother Jean-Pierre. Interesting, but without great import. My primary focus in this etude is that springy, fast detache in the middle of the bow, but it has the additional challenges of several passages in thumb position written in the tenor clef. I am making great progress in the thumb position passages, though they still aren't so secure that I can just play them after a break between practice sessions. That's not unexpected, and I know that I am making progress because it takes less time each practice to be able to play them again.
Having gotten that far, the next thing I have started working on this week are the bridges, the few notes that tie together what I can play to the tricky bit, or the transition from one tricky bit to another. An example.
The etude begins with four measures in first position followed by four measures of the same notes an octave higher played in thumb position, with the thumb on the half-string harmonic. The first note of the first passage is open string G followed by 4th finger D on the A string, and the first note of the repeated motif is harmonic G followed by 3d finger D on the A string. This week I have spent all of my practice time on this etude just getting the following steps:
Step 1: play open G with hand loosely in 1st position, play harmonic G with the thumb (get the location)
Step 2: play step 1 followed by 3d finger D on the A string (get the hand position)
Step 3: play the last few notes of the first passage (which includes a shift to 2nd position) then step 2 (get the shift)
Step 4: play step 3 then the next few notes in the second passage
Step 5: play both passages
It still amazes me how long it takes the muscles to be able to do this reliably, and then it amazes me again to see how reliable it is once that happens. This is what keeps me coming back to the cello.