Measuring phase-to-phase voltage with a multimeter involves connecting the positive probe to one phase and the negative probe to another, resulting in an RMS voltage reading that reflects the difference between the two sinusoidal voltages. Digital multimeters sample the voltage several times per AC cycle to calculate this RMS value, while analog multimeters may yield inaccurate readings at higher frequencies. The discussion emphasizes the importance of correct terminology, noting that "line-to-line voltage" is the appropriate term instead of "phase-to-phase voltage." Observations of voltage changes in a testing setup suggest that adjustments to one phase voltage can affect others, indicating potential imbalances in the system. Understanding the relationships between phase voltages and their implications for system balance is crucial for accurate measurements and analysis.