Bugbear, why do you ask this question when I just left for a long weekend on the camping without internet access? (hammer) :lol:
I have some clues, not a definite answer. I researched this quite a bit some years ago. First I tried to find which high pitch corresponded to what chipbreaker setting in tear out reduction, in order to be able to compare them. I found that a 60 degree pitch nicely matched a 0.1 mm setting of a chipbreaker with a 45 degree front bevel.
http://planetuning.infillplane.com/html/cap_iron_study_by_kees_van_der.html
Then I set up a planing machine with force transducers on our metal working lathe.
http://planetuning.infillplane.com/html/mechanics_of_chipbreakers.html I could meassure force in two directions, horizontal, the force you feel when you push a plane from behind, and vertical. The horizontal was an easy one, higher pitched planes are harder to push then double iron planes with a comparable chipbreaker setting.
The vertical force is more interesting. This force is a result of the shaving pushing down on the plane iron, and the reaction force is the plane iron pushing the shaving upwards and trying to pry it out of the wood: tearout. Here I found something interesting.
With increasing pitch the vertical force dissapeared quickly until it was about 0 in a 60 degree plane (always under assumption of this type of wood with this grain direction and this shaving thickness). This means that with increasing pitch, the shaving is being pushed upwards less and less, it only receives a forward push. In other words, the shaving collapes forward, and isn't pried up out of the wood, result: no tear out.
In a double iron plane this vertical force is also reduced but not nearly as much as in the high pitch plane. It went from -0.5kg in a 45 degree plane without chipbreaker to -0.4kg in a 45 degree plane with the capiron set at 0.1 mm from the edge. In other words, this is not nearly enough to reduce tearout in the same way as it does in the 60 degree plane. There has to be another mechanism.
My theory, and it is of course just a theorie, I can't really prove it. The shaving after being cut at 45 degrees flows up the face side of the iron and is nicely being pried upwards, probably wanting to be torn out in a major way. But then it suddenly meets a wall. The vertical face of the chipbreaker bevel! The upwards flow is hampered, the shaving meets resistance. Thus the upwards travel of the shaving is hindered, you could say that a force is pressing the shaving backwards, backwards into the wood. I think that this is the main reason why a chipbreaker is effective against tearout.
Another thing about this vertical force, I firmly believe that this is the more important factor in how we determine if a plane iron is still sharp. When the edge wears, a wear bevel forms, a rounded bulge under the edge. This bulge presses against the wood and pushes the iron upwards, counter acting the downwards pressure of the shaving. If this balance gets too much out of control, you have to press down on the plane harder and harder to keep it cutting. Thereby also increasing the friction under the plane making it harder to push forwards. This also explains why a jackplane can keep on going much longer then a smoothing plane: a thick shaving presses down on the iron much harder.
Hope this was kind of illuminating. Feel free to answer questions.