Paul, just the opposite as it turns out. When the fan is moving less air, it's drawing a lower amperage. With all blast gates closed, the motor/blower is idling with low current draw. When it's fully engaged (blast gate(s) open), it's drawing the most amperage and has the most loading. Some motor/blower combinations "require" some level of resistance on the output side to insure they don't "over perform", if you will. That's what the caution is about.
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The most expensive tool is the one you buy "cheaply" and often...