What specific stellar light shift is measured when applying the radial velocity method to detect orbiting planets?

The shift toward the blue or red end of the light spectrum.

The radial velocity method, also known as the Doppler technique, exploits the fact that both a star and its planet orbit a shared center of mass. As the star wobbles slightly due to the planet's gravitational influence, it periodically moves toward Earth and then away from Earth along our line of sight. When the star moves toward us, its emitted light spectrum shifts towards shorter, bluer wavelengths (blueshift). Conversely, when it moves away, the spectrum shifts towards longer, redder wavelengths (redshift). Detecting and measuring these subtle spectral changes allows astronomers to infer the presence and characteristics of the orbiting companion world, analogous to how the pitch of a siren changes as it passes by.