When a watch mainspring is wound, it contains a large amount of energy. Even when the spring is in its most relaxed state within the mainspring barrel, it still contains a lot of energy. When removed from the mainspring barrel incorrectly the spring will fly out and hit you in the eye. For this reason, only the correct tools should be used for removing and inserting the spring into the barrel. Having said that, in this post I will remove and install a pocket watch mainspring by hand using the incorrect tools. This should be the last time I have to handle mainsprings like this as I have a nice winding set coming in the mail...
To start, the mainspring barrel, the picture on the left is the underside (front of watch), the right is the topside (rear of watch).
Take a flat tool and pry the lid off the barrel, whilst applying light pressure to the outside of the barrel, so that the central arbor assists in raising the lid.
Watch that the lid does not fly off! Get the flat tool into the barrel and hold the spring down. Always keep pressure on the spring until you are sure that the arbor won't pull it out when removed.
Surround the barrel with as many fingers as possible, and slowly draw out the spring so that the spring slowly unwinds into your fingers.
Success! This spring is due to be retired, as the central notch that attaches to the arbor is fatigued and will likely break or fall off when wound slightly. However, I'll put it back inside the barrel anyway.
Start by lining up the outer notch of the spring with the notch in the barrel. Keep pressure on the spring to the lip of the barrel, whilst turning the barrel.
Success again! The old spring is reinstalled in the barrel. The downsides to this method are the incredible ease with which you can break/bend/generally mess up the spring (especially with smaller watch reverse coil springs and barrels), the ease with which arbors and barrels can be flung across a room and lost forever, and the grubby finger prints, oils and dirt that can be transported into the mainspring barrel (I should use finger gloves).