Many cellists, even advanced ones, have a fear of playing in the upper positions of the cello. Away from the comfortable neck position, where the thumb opposes the fingers, they feel ill at ease, even lost. The tone loses its resonance, and intonation suffers.
In Chapter Four of Cello Practice, Cello Performance ("Solutions to the Challenges of Cello Playing"), I wrote:
"Much of the common fear of thumb position results from difficulty transitioning there from the neck position. It sometimes helps to remember, as obvious as this sounds, that the string is just a straight line. Your job is to use the most economical arm movements possible to transition between different parts of this straight line with your fingers. Therefore, have your left arm on the same 'plane' as you travel from neck position to thumb position and back."
There's a neat exercise, invented by one of my teachers, that's like a reset button for your shifting technique. You make your hand into a loose fist shape, and you drum your knuckles up and down the strings of the cello. Notice how little you actually have to alter your arm's elevation in order to get this done. The idea is to adjust your arm's position as little as possible as you climb over the "cusp" of neck and thumb position. Here's what it looks like. (Click on the pictures to see the transition.)
You can use this exercise to plot your shifting whenever you have an awkward spot in a piece where you have to get from neck position to thumb position in a hurry. One such passage occurs in measure 88 of the first movement of Haydn's C major concerto.
There are many possible fingerings for this passage; I've shown one that I find pretty easy once my arm is positioned correctly. The fast tempo means that you have to get over the transition quickly, easily, and smoothly. Here's what my arm looks like when I do it. (Click on the pictures to see the transition.)
Sometimes part of the difficulty of this transition is getting your thumb up too. Many cellists find this run in the first movement of the Lalo concerto challenging because the thumb has to come out of its comfortable position opposing the fingers to play the A. (It's fine, in my opinion, to use a harmonic here.)
The trick to executing this smoothly and swiftly is to get your thumb "out" considerably before you have to use it. This can feel a little awkward when you're trying to keep your arm's elevation consistent. In these photos, I brought my thumb up while I was still on the F in the neck position. (Click on the pictures to see the transition.)
Need some etudes to work on this? Try Louis Feuillard's Daily Exercises (Schott), pp. 7-9. Be sure to keep the bow stroke strong and confident as you work on the shifts: as I wrote in Chapter One of Cello Practice, Cello Performance, "The efficient fingering and shifting of the left hand influences and empowers the efficient bowing of the right, and vice versa...All actions are mutually interconnected in the pursuit of cello playing that is both efficient and effective."