How the speed change works
With Keep the Pitch on, the tool runs FFmpeg's atempo filter. Atempo slices the audio into small overlapping windows, then stretches or compresses the gaps between them so the track plays back slower or faster while every note stays in its original key. It is the same idea a podcast app uses for 1.5x playback, just running locally instead of on a phone chip.
With Keep the Pitch off, the tool switches to asetrate instead. Asetrate relabels the sample rate the audio is read at without touching the samples themselves, so a slower rate stretches every waveform out, dropping the pitch, and a faster rate compresses it, raising the pitch, exactly the way a cassette or vinyl record sounds when you change its motor speed.