What we didn't tell you, and you didn't ask, is what the malted barley flour does. Simply stated, its purpose is to convert damaged starch to simple sugars. Damaged starch is starch in the flour that is damaged during milling of the wheat although wheat sprout damage can also result in high levels of damaged starch, maybe even excessively so. There are enzymes in the malted barley flour, called amylase, that work on the damaged starch to convert it to simple sugars. This conversion is helped by the fact that damaged starch can absorb more than three times the amount of water that undamaged starch can absorb.
The simple sugars are used by the yeast as food. Yeast can only feed on simple sugars. It can also feed off of sucrose, or regular table sugar, that might be added to the dough, but the table sugar has to be first broken down into forms of sugar that the yeast can use. But that takes time. So, if there is no added sugar in the dough, the yeast gets the bulk of its food from the breakdown of damaged starch. I say "bulk" because there are small amounts of natural sugars in the flour that help feed the yeast until the amylase enzymes do their job. Whatever sugars remain in the dough after the yeast has been fed are called residual sugars. These are the sugars that contribute to crust coloration at the time of baking. The Maillard reactions require these sugars to contribute to both crust coloration and flavors.
The above is a simplified version of how sugars are produced in doughs. For a more technical and elegant version of what happens, see the following discussion that was taken from the theartisan.net website:
Sugar Transformations (Rosada)
Simple sugars: The main simple sugars, glucose and fructose, represent about 0.5% of the flour. Yeast can directly assimilate them by penetration of the cell membrane. Simple sugars are transformed into alcohol and carbon dioxide by zymase, an enzyme naturally present in yeast cells. Because of this easy absorption, these sugars are the first ones used in the fermentation process. Their consumption takes place during the first 30 minutes or so at the beginning of the fermentation process.
Complex sugars: The two main types naturally present in flour, saccharose and maltose, represent approximately 1% of the flour. Because of their complex composition, these sugars will be used later on in the fermentation process. The lapse of approximately 30 minutes at the beginning of the fermentation period is necessary to achieve their enzymatic transformation into simple sugars. The enzymes involved are saccharase, which transforms saccharose into glucose and fructose, and maltase, which transforms maltose into glucose.
Very Complex sugars: The main very complex sugar is starch, which represents about 70% of the flour content. Two types of starch are found in flour: amylose and amylopectin. Amylose is degraded by the enzyme beta amylase into maltose, and in turn the maltose will be degraded into glucose by the maltase enzyme. Amylopectin is degraded by the alpha amylase enzyme into dextrin, after which the dextrin is degraded by the beta amylase into maltose. This maltose will them be degraded by the maltase into glucose.
The simple sugar, glucose, obtained during these transformations is used by the yeast to generate carbon dioxide and alcohol. During the fermentation process, most of the starches used are the ones damaged during the milling process. Because the particles are damaged, they can easily absorb water during the dough making process. This water contact triggers the enzymatic activity. A non-damaged particle of starch will only retain water at its periphery and not inside the particle itself.
To the above, I will also note that softer white flours, such as those milled outside of the U.S., including the Caputo 00 flours, have less damaged starch than our domestic flours. To extract more sugars out of such flours, you would have to damage some of the starch. Of course, that is not done. The flours remain unmalted.