THE LEFT THUMB.
The big difference with the left hand is that the height at which the entire arm is held is much lower as the fingers are playing on the bottom half of the strings. The elbow and shoulder are in a more relaxed position. However, the alignment of the wrist and elbow remains the same in the middle of the bass register, but as the hand drops and stretches towards the strings at the bass end, the hand tends to angle down from the elbow level, and as the left hand plays right up into the treble register, as it frequently must, the wrist level must lift above the height of the elbow.
The other really important difference is that the left thumb is your mainstay on the pentatonic row, as it conveniently reaches upwards to play the “black notes”. Initially this will not concern us and we will be playing only on the diatonic row. However, it does effect the closeness of the placement of the tip of the thumbs to the cusp. Where on the right hand the thumb is placed at the greatest distance from the cusp of all the fingers, with the left hand it must be placed as closely as possible to the cusp, and indeed, you must get used to the thumb being able to play in the spaces between the E flat and F sharp strings and the B flat and C sharp strings.
Keeping this in mind, and beginning with any string in the middle of the bass register, repeat what you have already done with the right thumb including the neural patterning.