Maths adds a bipolar voltage offset to any signal by using CH.3 attenuvertor as an offset control

To add a DC offset (+/-5V) to a signal using Maths: patch the signal to CH.2 Signal IN, use CH.2 Attenuvertor for scale and inversion. Patch dummy cables to CH.1 and CH.4 outputs to remove their contributions from SUM. Use CH.3 Attenuvertor to set the offset amount (range approximately +/-5V) with nothing patched to CH.3 Signal IN. Take SUM OUT as the offset signal. For +/-10V offset, swap roles of CH.2 and CH.3. Useful for biasing envelope outputs, setting comparator thresholds, or shifting modulation ranges.

Examples

Signal → CH.2 Signal IN. Dummy cables → CH.1 and CH.4 first outputs. CH.3 Attenuvertor set to desired offset level. Take SUM OUT.

Assessment

What parameter sets the direction of the DC offset, and why are dummy cables needed in CH.1 and CH.4 to make this patch work correctly?