It tricks the phone circuitry into "high impedance" aka high gain mode. The threshold is roughly 50 ohms (sometimes it takes a bit more to trigger) and you would get there by taking nominal impedance of your headphone + the adapter rating. Honestly it's only valuable to me for planars as everything else get's plenty of gain in automatic. There are some that mistakenly believe you don't get use of the full quad DAC if you aren't in "high impedance mode". This is incorrect, the only thing you are affecting is output voltage.