Clock Let Down Tool
The clock let down tool set of 4 comes with a handle and 3 chucks that fit just about every winder arbor size. Put the chuck into the handle and release the mainspring in a slow manner, carefully controlling the release with the hand grip. This is done by putting the tool onto the key post arbor and gently releasing the click.
While doing this, the fist is around the handle. This allows the power to release and let down with gentle care. Of course the other way of doing this is to try and do it with the key. This is not advisable and usually ends bad for the person's hand and the clock movement.