Three-Build System - Firm Starter

You must be reading Peter Reinhart, as he is the only author who used the term "barm," which is defined by Wikipedia as:

Barm is the foam, or scum, formed on the top of liquor (i.e. fermented alcoholic beverages such as beer or wine, or feedstock for hard liquor or industrial ethanol distillation) when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, has also been made from ground millet combined with must out of wine-tubs [1] and is sometimes used in English baking as a synonym for a natural leaven.[2] Various cultures derived from barm, usually Saccharomyces cerevisiae, became ancestral to most forms of brewer's yeast andbaker's yeast currently on the market.

"Barmy" is also British slang for "crazy", comparing the foamy texture of barm to the perceived emptiness of such a person's head.

What you're doing essentially is building a firmer starter from your original culture.   I've found there's a subtle difference in the taste of sourdough bread which uses a firm starter, compared to my normal starter, which is about 75% hydration (my firm starter is 64%).

I admit that I flunked Reinhart's seed culture and barm chapters.  It looked extremely complex and wordy to me so I switched over to Hamelman, whose instructions for building a levain I could easily follow.

Essentially, I think you are allowing the starter to propagate and grow. A fair number of SD revipes seem to do this.