I saw a teardown of a reel and noticed a part inside the spool:
Does anyone know what the spring does?
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The metal tab is called the click tongue, and the spring holds the click tongue against a sprocket on the reel spindle. Together they are responsible for the clicking sound a reel makes when line is pulled off against the drag, for example, when fighting a large fish. If a fish removes line rapidly, the individual clicks sound more like a continuous whine, a coveted sound to anglers known as a screaming drag.
There are other mechanisms in use in some reels, but I've seen this on Penn reels before. This image search link shows a number of variations.