Set the tier to Easy — the compression is heavy and obvious (20:1 ratio, −30dB threshold). Press Play, then use Space/D to toggle between sides A and B. Decide which one sounds compressed and click A or B.
As you get more accurate, move to harder tiers where the compression becomes subtler and harder to detect.
Compression reduces the dynamic range of a signal — loud parts get quieter, making the overall sound more "even" or "controlled." The compressed side will typically sound:
More sustained — transients (the initial punch of a drum hit) are reduced, so the tail of the sound is relatively louder. The sound feels "pushed forward."
Less dynamic — quiet moments are closer in level to loud moments. The track feels "flatter" or more consistent.
Pumping or breathing — on heavier settings, you may hear the compressor "letting go" between loud hits, causing the level to rise and fall rhythmically.
The uncompressed side will sound more open and punchy in comparison — transients hit harder and there's a greater sense of dynamics and space.