This tool is specifically designed moving and setting a sound post. The pointed end of the S-shaped setter is designed to pierce and position the sound-post inside the instrument and the other side is designed to pull or push in or out or tap from side to side. The position of the sound post inside a violin is critical, and moving it by very small amounts (as little as 0.5mm or 0.25mm, or less) can make a big difference in the tonal quality and volume of an instrument.
Soundpost adjustment is as much art as science. A good starting point for a sound-post is to have it a half thickness of the bridge behind the bridge towards the end-button also in line with the center of the E string side bridge foot. From this point, moving the sound-post closer to the bridge should increase the volume and brilliance of the sound while moving it away from the bridge should decrease the sound and less brilliance. Moving the post towards the f-hole/E string side, usually intensifies the higher register giving a brighter tone quality where as moving it away from the f-hole towards the center intensifies the lower register giving a darker tone quality.
To adjust the sound-post, it is best to loosen your strings, especially the E and A to avoid fraying and for easier movement. While being careful not to scratch the varnish or damage the edges on the f-hole carefully insert the non sharpened end into the hole and begin by lightly tapping the post either on the bottom or top, keeping the post as straight as possible and only moving with minimal movements while looking at the post. Alternate the taps from the top and bottom keeping it as straight as possible throughout the process. Tap almost directly in the desired position with a slight angle towards the f-hole to keep the post from sliding down and falling.
Adjust the sound-post at your own risk. There is always a risk that you will knock the sound-post loose and need to raise it again. Doing so is very frustrating the first time and requires calmness and patience. With experience one can become very proficient and confident to quickly move and set the post in the exact desired position. Once you have found the optimal position it is best not to touch the post again even if the tone seems to be changing. Adjusting the post too often can become addictive and even wear the instruments top and bottom plate causing a need for expensive repairs.