This is the commonly used fingering of
1 2 1 2 3 4 for the chromatic scale.
Notice how the thumb reacts to the change from the first 1-2 to the second 1-2 but it does not shift with the hand. This is because the shifting distance in the chromatic scale is so small that we do not need a proper shifting action.
You can also practise this with chain bowing (overlapping bowing, where you bow across the shift).