The Portia Street Stomp uses a wonderfully low inductance steel core transformer with quadrifilar windings (four identical windings). This allows for a variety of configurations to get a wide range of tones out of the both the transformer itself and the driving circuit.
primary
secondary
The Portia Street Stomp allows you to vary the tone in three ways: 
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The transformer “Ratio” switch selects between four configurations. Two configurations are effectively 1:1 but use 1 or 2 windings to increase or decrease the total inductance (which will enhance the behavior of the “Primary” knob). In 1:2 and 2:1 mode there will be a 6 dB step up or step down respectively, and the load being presented to the source amplifier will also change.
 
The “Primary” knob allows the source impedance presented to the transformer to be changed continuously, which increases the harmonic distortion of the passive circuit and slightly changes the frequency response.
 
The “Secondary” pot allows the load across the transformer secondary to be varied. This will change the current drive of the source amplifier. A higher load will increase the current demand at a given signal level, which will cause more distortion from the source amplifier. The load pot will also interact with the transformer windings to change the overall tone.