It is sometimes necessary to adjust the regulating pins on the regulating unit to ensure the hairspring is beating in between the pins and not over to one side.
This is important because the point at which the spring hits the regulating pin denotes the active length of the hairspring, thus affecting timekeeping. If the pins are too wide, the watch believes the timing point is the stud, thus theoretically lengthening the hairspring. If the pins are too close together they will pinch the spring and prevent it from breathing properly, which is s no good either.
It is possible to diagnose the position of the spring in regards to the regulating pins without even looking at it if you know the signs to look for.
Here’s an example in which a hairspring is leaning against the inner regulating pin:
At very low amplitude the spring will not have sufficient power to move away from the regulating pin and thus the time will be taken from that point, causing a gain. When amplitude increases, the spring will move away from the pin, but not so much that it touches the other (the outer) pin. The watch will then take its timing point from the stud, during this phase, which will lengthen the active length of the hairspring and cause a significant loss. When the watch is fully wound and at full amplitude, the watch should have enough power to move the spring from pin to pin, shortening the active length and causing the watch to speed up.
This spring position could make the watch appear a solid time keeper when fully wound and, in an automatic watch worn daily, and thus providing consistently high torque from the mainspring to the escapement, could function adequately in this state indefinitely. However, should the mainspring uncoil due to the low activity of the wearer or the watch being removed from the wrist, the timekeeping would go haywire as explained.
