A 24-volt DC solenoid valve inside the compressor provides the means to modulate the compressor.
When the solenoid valve is energized the compressor is in full-load and when de-energized the compressor is in part-load. When the solenoid is energized a 3-way solenoid valve provides pressure to a lift ring assembly that is used to open and close the scroll modulation ports. A single-speed motor continues to run while the scroll modulates between the two capacity stages.
Two-stage podcast – https://hvacrschool.com/podcasts/copeland-2-stage-zps-scroll-compressors/