This variation is wonderful for developing general coordination and accuracy as well as preparing us for the many similar passages in the violin repertoire.
Make sure that the left arm is released and available to move the hand and fingers freely so that the finger fall on each chord is clear and secure.
Think about the difference between the open hand position for 5ths, 6ths, 7ths and 8ves and the compact position for 2nds, 3rds and 4ths. For instance, the chords of the first 5 bars use the intervals of 5ths and 6ths, intervals which like flatter fingers and consequently a left-arm position that is more to the left and more under the violin. When we change to a 4 – 3 third in bar 6, we need to let the arm move a little to the right so that the shorter 4th finger can reach the D string without covering the A string. These movements of the left arm can be consciously programmed in and exaggerated in practice, but of course when we perform they are minimal and automatic.
The string cross action across 3 strings actually needs greater control than for crossing 4 strings, since we need to control the amplitude of the action more when just crossing 3 strings. For both 3 and 4 strings the whole arm needs to move freely from the shoulder joint.
If we want to extend the length of the bow stroke (to play more loudly) then we can use the forearm from the elbow joint.