Hollow Building
Hollow building a bamboo fly rod means exactly what you'd expect: Material from the inside of a bamboo rod is removed leaving open spaces within. The purpose of removing the material is to make the rod lighter in weight and make it rebound and recover quicker with less bounce and excess vibration.

There are many methods of hollow building. Most methods have disadvantages, the most severe being a significant reduction in the overall strength of the rod. I have experimented and made rods with every type of hollowing I could find - straight-cut, fluted, star-hollowing and scalloped.
Solid Cross Section
Straight-cut Hollow
Fluted Hollow
"Star" Hollow
After testing these rods I have come to the conclusion that the best method, for both improvement in action and overall strength, is a method first described and developed by EC Powell.

Simple scalloping involves removing small ovals of the softer, inner pith of a beveled and shaped bamboo strip. By mirroring the ovals on each of the 6 identical strips of a hexagonal rod (or 4 strips for quads) you end up with small hollow ovals inside the finished rod. This method of manufacture is, as a friend put it, the closest to "OEM specs" of a bamboo culm. In other words, like a culm of bamboo, there are hollows that are reinforced by dams throughout the length of the culm.

Of course that is not the end of the story. The next step is to vary the length and wall thickness of the scallops, or hollows, inside the rod in order to improve specific desirable qualities. Longer, thin wall scallops are better suited for the thinner tip portions of a rod while shorter, thicker walled scallops keep butt sections from becoming weak or too soft.
I have designed a complex mixture of scallops with different wall thicknesses, dam thicknesses and lengths and optimized their layout for each section of a rod from under the grip into the tip. This gives, what I believe, is the greatest benefit of hollow building while retaining the strength and feel of bamboo.
This photo shows a side view of a single bamboo strip that has been scalloped (approx. 1" long).
This photo shows what half of a rod (3 strips from a hex rod in this case) looks like when scalloped and assembled.