That's sorta what I was getting at- if the field comes from a coil instead of a permanent magnet, then the power output can be boosted by (1) spinning it faster and (2) boosting the field current. Therefore in a high-rpm, low-load scenario cutting back the field current can reduce the output. One other advantage to that- reducing the field current reduces the mechanical load, giving more horsepower to that spinny thingie up in the front.