This signal tests the ability of a decoder to handle overshoots above full scale limits in the bitstream, such as will be produced by encoding highly compressed, hard limited or clipped ultra-hot CDs, particularly with "0 dBFS +" mastering.
On a decoder or player with a safety limiter or unsaturated floating point output, you should hear a pure tone that increases in loudness (with a brief transient when the level changes every two seconds). On a simple decoder (as on mobile devices) you will hear clipping until it becomes so strong the waveform just becomes a square wave and you hear additional harmonics.
Input signal to specially modified encoder. This signal is 12 dB lower than the bitstream values produced by the modifed encoder. The bitstream will make a reconstructed signal in a standard decoder that is 12 dB higher.