Note: For Left Hand Thumb positioning, check out the previous installment.
When there is excess tension in the hands, the source is often improper use of the thumb and its corresponding muscles. The thumbs of both hands, just like all the fingers, should use minimum effort and pressure to achieve any particular technical goal.
When in standard “neck position” (i.e. not “pivoting,” extending, or “thumb position”) the thumb should provide minimal counter-pressure, to the fingers, against the neck. The pressure should be directed through the side/tip/pad of the thumb and in toward the fingertip currently in use.
This is done primarily through “curling” the thumb inward, or moving it up or down (i.e. more or less perpendicular to the plane of the hand) and allowing for free movement of all the thumb joints. The thumb should not provide counter pressure by “clamping” the thumb toward the palm of the hand, as if attempting to touch the entire thumb to the palm of your hand.
You will know you are using your thumb correctly to provide counter-pressure if it seems that the muscles required for the thumb to move are in your forearm. Depending on the specific movement you are making with your thumb, the muscles involved might be on top of the forearm, or on the bottom side. If you feel the large muscle at the base of your thumb, in your hand, as the source of the movement, you are moving your thumb incorrectly.
This is not to say that this large muscle in your hand will not be engaged in some small, responsive way, but the movement of the thumb should not originate here. Overuse of this specific muscle is to be avoided at all costs. When it is used it should be released as soon as possible. Consistent or excess tension here, in the right or left hand, is not beneficial and the source of many problems.
At all times, the amount of pressure provided by the thumb should as much as needed and as little as possible. Even when applying counter pressure with your thumb and fingers, you should also be able to shift (or play a glissando) with minimum friction created between the thumb and the neck. A glissando should feel free and easy. If you “squeak” or otherwise stumble when you try to shift, then you are probably pressing to hard with your thumb. See my previous article on minimum finger pressure for more, as the less the finger pressure, the less counter pressure required by the thumb.