I was soaking in my hot tub this afternoon and I was thinking about what I know about borates and came up with this theory of how they stabilize pH. The claims made for borates have proven to be true in actual tests in several pools, including my own. They include reduced sanitizer demand, more stable pH, and "sparkling" water. The reduced sanitizer demand is easily explained by their algaestatic properties but the actual reduction in sanitizer demand is much more than expected so perhaps something else is going on here also. The "sparkling water" effect could be possibly explained by the borates not allowing any nascient algae blooms so the water is clearer than without. By lowering one of the things necesary for algae growth it just won't grow and that is going to be more effective than killing it after it starts with the sanitizer. We all know dead algae will cloud a pool so perhaps if it never gets a chance to start growing in the first place the water will have more "sparkle" then water that is relying on the chlorine to kill any algae that starts to grow. Even a tiny amount of dead and oxidized algae might be enough to deaden the "sparkle" of the water.
First we need to look at the way that borates act as an algae preventative. They do this by reducing the amount of carbon dioxide in the water. Carbon dioxide is necessary for photosynthesis. I suspect this occurs because any H+ ions in the water react with the borates and form boric acid instead of reacting with the carbonates and bicarbonates to form carbonic acid. Carbonic acid is simply carbon dioxide dissolved in water. I must stress that this is only supposition and I have not looked at the chemistry involved yet nor investigated if the formation of boric acid is more likely than the formation of carbonic acid when borates are present.
If the above IS true and the secondary borate/boric acid buffer in the water is more active than the primary carbonate/bicarbonate/carbonic acid buffer then less carbonic acid will form when pH is dropped and that will lead to less outgassiing of carbon dioxide. The outgassing of carbon dioxide is what causes pH to rise. If there is less formed in the water then the pH will not rise as fast. This could explain the more stable pH that many, if not all, users of borates have reported. This is in line with Chemgeek's recommendation of keeping the pH at 7.6 and lowering the TA to help stabilize pH by lessening the outgassing of carbon dioxide when borates are not used. By doing this you are minimizing the formation of carbonic acid in the water.
I would appreciate any feedback on my theory since the actions of borates have never been properly explained anywhere that I have been able to find.