NLP anchors are basically about using a conditioned response.

Pavlovs dogs salviate in response to the bell. The bell produces a bodily reaction. If you want to control your own state, then carrying a bell around with you is impractical. Visualized symbols are more practical because you can trigger them without needing to do anything externally.

Going through the intermediate step of the bell or a visualized symbol would be an indirect way to change bodily sensations. Biofeedback allows a practitioner to learn to have more direct control over a variable.

In NLP the creation of an anchor is usually done with a coach leading the process for another person. It's not something that's as easily done when you are alone. On the other hand once the anchor is there you can use it whenever you want. But you actually have to remember to use it which people often fail to do in high stress situations. I'm not sure whether it's useful to link you to some online description.