In the diagram, the blue indicates internal air volume.
The tuning plunger allows for interior volume adjustment. Before this innovation, if a player wanted to lower the overall pitch (fundamental or key note) of his/her ocarina, there were two main ways of doing this. The most common method used is by altering breath pressure (blowing harder or softer). Another method is by "shading" the notes (partially covering the voicing with a lip or finger).
With a "sweet potato" ocarina, this second technique may be nearly impossible, because the mouthpieces on this style are usually fairly long and the fingers are being used to cover the 10 finger holes. The player would be unable to compensate for changes in temperature and humidity.
The hollow plunger tube, when pushed inside the ocarina, decreases the interior volume by a small amount due to the thickness of the tube walls displacing air. When the plunger tube is pulled out, it is no longer taking up space inside the ocarina. Plus, it is adding volume to the interior. The pulled-out hollow tube's inside is an extension of the inside volume of the ocarina. With more interior space, the lower pitched the ocarina will be.
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