While it is an expression of at least a moderate amount of manual dexterity, thumb twiddling is frequently used as an example of a useless, time-wasting activity. It is equated with other activities that are also considered useless by the speaker. Therefore it is most often used as a synonym for an activity that will take the attention of the performer, but will not yield any useful results.
Thumb twiddling can be seen as an expression of which hand is dominant with the individual. The weaker handed thumb will "follow" the dominant handed thumb in the activity.
Another variation, which is very infrequently used, is for two people to participate in the activity, one contributing the left hand, and the other the right hand. This is often very difficult.
Most people tend to twiddle their thumbs in the direction where the thumb currently at the top goes towards the fingers. It is also possible to twiddle one's thumbs in the opposite direction, with the thumb at the top going away from the fingers, but this tends to feel awkward in the thumb bones and is thus slower and more stressing.
Twiddling the thumbs in opposite directions from each other is somewhat more difficult than regular thumb twiddling. Often, this is part of what is known as the "Hat Trick". After twiddling the thumbs in the opposite direction, one might "twiddle" their forearms in the opposite direction. Then finally, they rotate each of their arms in the opposite direction using the shoulder as the pivot point of the corresponding arm's rotation. The Hat Trick is regarded as difficult.