It's definitely an impressive achievement by the US Department of Justice and US Marshal Service. Most of the details of the program are kept a secret for obvious reasons, but a few things are known. For one, all participants receive completely new identities with authorized documentation, which would make it near impossible to track that person.
The service also gives witnesses full time security while in a high-threat environment, and witnesses receive money for housing, medical care, and essentials. Participants also receive job training and employment assistance.
The program is open to anyone who testifies for the prosecution and is completely voluntary, but since you have to completely cut ties with your previous life, it's generally only used by people in real danger. Witnesses are permitted to leave the program at any time as well, though this is discouraged.
