What's inside this fishing reel?

Question

I saw a teardown of a reel and noticed a part inside the spool:

https://youtu.be/_NKVN9fRD4k?t=327

Does anyone know what the spring does?

Answer 1

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.