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To the best of my knowledge:
Wheel speed is based on the ratio of MG2 and ICE(+MG1) RPM, which are both connected to the wheels via a planetary gearset. During normal driving MG2 or the ICE can be entirely stationary, or MG2 can even spin backward, to get the desired wheel speed. As the RPM on one goes up, RPM on the other needs to decrease, to maintain the same road speed. You can't get max horsepower out of both simultaneously because of the power bands for each - if the ICE is at redline, MG2 will need to be spinning much more slowly or even backward, preventing it from producing peak horsepower at any reasonable road speed. Additionally, MG2 may also not be able to produce peak horsepower without additional power sapped from the ICE via MG1 - the battery alone in these types of hybrids often isn't adequately large to provide peak energy requirements of the electric motor.
I can't speak in any detail about the system because there's relatively little I've seen published on it, but I have a decent understanding of how series-parallel hybrids with planetary gears.
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