Title 18 of the United States Code

Title 18 of the United States Code is the main criminal code of the federal government of the United States. It deals with federal crimes and criminal procedure.

This statute covers a specific way to satisfy the Fifth Amendment (right to silence as a form of protection against self-incrimination) to the Constitution, but still force witnesses to testify. Basically, if a witness—whether in a federal court such as a United States District Court or in testimony before a Congressional subcommittee—refuses to answer questions and pleads the 5th, the presiding officer can use the provisions of Title 18 Chapter 601 to forcibly compel the witness to answer the questions. Since this would violate the 5th amendment rights of the witness, the statute requires that the presiding officer must mandatorily preserve those rights, by guaranteeing the witness immunity from prosecution for anything they might truthfully say under such compulsion. (The witness is being compelled to answer the questions truthfully—if they lie, they can be tried in court for perjury, but as long as they tell the truth, they are immune from being personally prosecuted for anything they might say—which is the reverse of the usual situation, where anything you say can and will be used against you in a court of law.)

Actually giving a particular witness guaranteed immunity as a means to compelling their testimony is somewhat involved; the details of how it is done vary depending on the particular branch of government hearing the testimony. If the witness is testifying before an agency (includes Army/Navy/AirForce/VA/DOD/HomeSec/StateDept, FCC/FTC, DOT/NTSB, DOE/NRC/COP/DeptOfTheInterior, SEC/CFTC/FedBoard/FDIC, NLRB/LaborDept/CommerceDept/AgDept, DOJ/Treasury, and many others), the presiding officer for the agency needs approval from the federal Attorney General before they can grant a witness immunity and compel testimony. In court cases, the federal district attorney (for the particular federal district court which has jurisdiction in the case) needs approval from either the federal attorney general directly or from a specific set of the federal attorney general's underlings. In the case of testimony before congress, the body hearing the testimony must vote on whether or not to give immunity as a means to compel testimony, before getting a federal district court to issue to compulsion order; for a subcommittee, two-thirds of the full membership must vote affirmative, whereas for testimony before an entire house of congress a simple majority of members present voting affirmative is acceptable. Although congress must notify the federal attorney general 10 days in advance of submitting their request for compulsion to the federal district court, the AG cannot veto the order (but they can at their option instruct the federal district court to delay issuing the compulsion order for a period up to 20 days total).

...
Wikipedia