There are two high pass filters. One of them is a series leg, and attenuates the low frequencies being passed through to the output. This is the high frequency pass through that most people think of. The other is a shunt leg, and attenuates the low frequencies being shunted off to ground. This is passing the highs to ground rather than to the output, so it actually is a high frequency subtracter. Notice in Mikes plots that unlike a simple highpass circuit, the highs are boosted (or cut) more and more over a range of frequencies, then the boosting (or cutting) levels off. This is the effect of the shunt leg cancelling out the series leg. Adjusting the pot adjusts the ratio of the the series and shunt leg signals being summed into the output. When the pot is centered, the ratio is 1:1 and the freq response is flat.
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