Generally, young officials (usually in their thirties) of low or middle rank were appointed as secret royal inspectors because they had to travel long distances for an extended period of time.
For their secret mission, they received a letter of appointment (bongseo, 封書, 봉서), a description of their destination and mission (samok, 事目, 사목), and a "horse requisition tablet" called mapae(馬牌, 마패), which they used to requisition horses and men from a local station run by the central government.
After the secret surveillance was completed, they revealed themselves by presenting mapae or bongseo and inspected the magistrate's office and records (Chuldu, 出頭, 출두).
If they found cases that were unjustly judged, they presided over a retrial (Eosa-Jegim, 御史題音, 어사제김) to redress wrongs.
Many secret inspectors, such as Jeong Yak-yong, suffered political repercussions in the royal court from the magistrates they had prosecuted during their missions.
Famous figures who served as secret royal inspectors include Jo Gwang-jo, Yi Hwang, Jeong Yak-yong, and writer Kim Man-jung.
Many legends about Park Mun-su as the avenger for the people exploited by corrupt officials have passed down through folk tales; there are 300 such stories.