This tests how sensitive the DUT is to power supply osillation or how well the DUT is isolated to this, so this is a max dB test. The specified tone is injected on the power supply and the DUT channel input is set to 0V. Digitize the output of the DUT, compute the FFT and check the frequency bin for the input power supply tone.
Consider the non-inverting op-amp circuit below with dual supplies. The gain is set close to unity and the input is a 1V Vpk 1KHz tone. In Figure 2, the positive supply (V+) is maintained at a DC level of 15V with no tone. The FFT of the output, Vout is shown in Figure 4.
In Figure 5, the DC offset on V+ is set to 15V, but a 5V 10KHz tone is injected on top of it. We can see the 10KHz tone show up in the FFT of the output, Vout in Figure 6.