Apart from those times when our left-hand thumb is being used in thumbposition, both thumbs are more or less invisible, hidden from view under the hand, behind the cello neck (for the left hand), and tucked away against the frog of the bow (for the right hand). The fact that we can’t see them doesn’t however mean that they are unimportant: quite the contrary! Just like for our psyche, it is the bits that are hidden away that often cause us our greatest problems! Part of the reason for this powerful negative potential is that their invisibility (and their silence) means that we can easily be unaware of both what they are doing and of the influence that they might be having.
If this influence is positive, then it doesn’t really matter that we might not be aware of what our thumbs are doing. It is when this influence is negative that it becomes important to be aware of how our use of those hidden thumbs might be contributing to our problems at the instrument. It is only when we have this awareness that we will be able to make an accurate diagnostic of our problem and thus convert this destructive influence into a positive helpful one.
And we should definitely not underestimate the destructive potential of these simple muscular labourers, working away silently in the background. A tense, stiff, rigid, inflexible righthand thumb can have a truly terrible effect on our bowhand flexibility, affecting negatively all aspects of our bowing but most especially our sound quality, legato bow changes, spiccato, and control of bow starts/stops (see Bowhold). And a left-hand thumb that clamps tightly onto the back of the cello neck will have a catastrophic effect on all aspects of our lefthand technique (see Finger Pressure).
So, in the same way that our “unconscious” mind has a powerful effect on our psyche, our hidden thumbs can have an identically powerful effect on our cello-playing. Fortunately it is a lot easier to become aware of the way we are using our thumbs than it is to become aware of our unconscious psychological processes. And the correction of our thumb-use is also considerably easier to achieve than the modification of our unconscious psychological processes! But in both cases the awareness (and consequent modification) require shifting our attention from what is occurring “on-stage” to what is occurring “backstage”, in the machine-room as it were.