When adjusting balances with the Etachron regulating system, the hairspring should be centered perfectly between the two regulating pins when the balance is at rest at the dead point. The pins should be closed (by rotating the regulator block so that the outside pin moves away from the stud) until there is almost no deviation between the vertical and horizontal rates on the timing machine. The hairspring must be able to move freely between the pins, but the best rates are usually achieved when there is a very small gap.
During the supplementary ascending arc of the balance wheel, the hairspring coils expand outwards. As it expands against the outer regulating pin the curve of the spring flattens, allowing a very small amount of hairspring to slide between the pins, effectively lengthening the hairspring. This lengthening is offset by the fact that the outside pin is positioned further from the stud.
During the descending arc of the balance wheel, the hairspring coils contract inwards. As it contracts against the inner regulating pin, the curve of the spring tightens allowing a very small amount of hairspring to slide between the pins, effectively shortening the hairspring. This shortening is offset by the fact that the inside pin is positioned closer to the stud.