CIE color matching functions empirically measure how much RGB is needed per spectral wavelength

In the 1920s, Wright and Guild independently ran experiments where observers adjusted red, green, and blue lights to match each visible spectral wavelength in 10 nm steps. The resulting rgb color matching functions define, for each wavelength, the signed amounts of reference primaries needed. Negative values indicate wavelengths where one primary must be added to the target side. These functions were the empirical basis for CIE XYZ (a linear transform was applied to eliminate the negatives and separate luminance).

Examples

At 570 nm (yellow): 0.167 units R, 0.171 units G, −0.001 units B needed to match the spectral color.

Assessment

What do negative values in the CIE RGB color matching functions represent physically, and why does the CIE XYZ transformation eliminate them?