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First semihollow vs. hollow: seems to me that "semihollow" was a term first used to describe E.C. Powell rods because the cedar core to which the bamboo was laminated was removed at intervals, not down the entire length of the rod. That left solid dams spaced along the shaft in between the hollows - thus "semihollow" in contrast to other rods that were hollowed continuously along the length of a rod section. The differences between hollow and semihollow per se, at least as it affects action and casting, isn't nearly as great as the difference between whether a rod is hollow in the butt only or also hollow in the tip and mid sections.

I know it's been said before, including in this thread, but it's not the removal of dead, static weight that changes rod action and recovery, it's the location of the weight removed. Adding some fat lead wire to a rod just above the grip won't change rod action an iota. Add that same amount of weight to the tip section and it will make the rod slow and soggy. The reverse happens when you remove weight. A small amount of weight removed in the tip section of a rod has a far greater effect than the same amount removed from the butt. When the rod is held motionless, where you add or subtract the weight makes no difference - weight is weight. When you begin to move the rod, where that weight is added or subtracted makes a big difference!

In addition, when you remove lower-strength pith, you are removing weight without losing a proportional amount of strength - improving the strength-to-weight ratio of the rod material. On top of all that, it's just a mechanical fact that the fibers closer to the center of the rod are less important than those on the outside of the rod. Those exterior fibers (that happen to be bamboo's power fibers) do all the stretching and compressing when the rod alternately bends forward and back as we cast and are the most important. The fibers, or in reality pith, in the center are on the neutral axis that neither compresses nor stretches and add little or nothing to the rod's strength. Getting rid of them is a benefit except for the crush resistance they might provide (thus one reason for dams).

What this all means is that the notion that hollowing doesn't have much effect on lightline or shorter rods and is really only helpful in longer, heavier rods is a myth. Yes, it does have a greater effect in longer rods, but in an 8' 4 wt. rod it can have a huge beneficial effect on action and recovery. Removing deadweight in the butt makes the rod feel lighter, but doesn't do much for the action. That's not really what hollowing is best for. It's removing weight in the upper rod sections that has the effect on action, feel and speed of recovery. When someone says hollowing makes no difference in lighter or smaller rods, they are either talking about hollowing in the butt only or haven't given it a fair try. It's hard for a decent caster NOT to notice.

The true test, of course, would be to cast rods that have been hollowed in the tips against the same rods that have not. One would have to be a rodmaker (or a good friend of one) to do exactly that, but I think casting a bunch of hollow rods against some similar solid rods would give anyone the feel of the difference. The myth about hollowbuilding only making rods lighter dies hard, it seems, but as more builders are trying it and seeing what it does, the myth is at least slowly losing its acceptance.