It is widely known that the spindle speed variation (SSV) is an effective technology for chatter suppression, especially in the turning or boring process. Its simple optimal design, however, is not a simple task. In the past, certain research works considered the chatter onset from the perspective of process energy balance in a vibration cycle. The phase shift between previous (i.e., outer modulation) and present vibrations (i.e., inner modulation) of chatter is a key factor in the process energy balance. The SSV can be conceptually interpreted as a technique that continuously perturbs the phase shift between the inner and outer modulations, thereby changing the process energy balance. Simply put, the chatter energy can be controlled by applying the SSV to suppress the chatter. This study investigates the correlation between the process energy balance and phase shift behavior in the sinusoidal SSV through numerical energy simulation. The results indicate that the phase shift at the maximum spindle speed is an important factor to minimize the total energy balance (i.e., to dissipate the chatter energy) in the SSV cycle. This probably corresponds to the fact that the beat vibration tends to occur near the maximum spindle speed in the SSV. The insights gained from this study are anticipated to serve as a guideline for shaping the phase shift profile in the SSV to effectively suppress chatter vibration.