It is blanket protection from testifying during any phase except if you've been given immunity. Then your testimony cannot be used against you and you can be compelled to testify.
"If the government gives an individual immunity, then that individual may be compelled to testify. Immunity may be "transactional immunity" or "use immunity"; in the former, the witness is immune from prosecution for offenses related to the testimony; in the latter, the witness may be prosecuted, but his testimony may not be used against him. In Kastigar v. United States, 406 U.S. 441 (1972), the Supreme Court held that the government need only grant use immunity to compel testimony. The use immunity, however, must extend not only to the testimony made by the witness, but also to all evidence derived therefrom. This scenario most commonly arises in cases related to organized crime."
http://en.wikipedia.org/wiki/Fifth_Amendment_to_the_United_S...
"If the government gives an individual immunity, then that individual may be compelled to testify. Immunity may be "transactional immunity" or "use immunity"; in the former, the witness is immune from prosecution for offenses related to the testimony; in the latter, the witness may be prosecuted, but his testimony may not be used against him. In Kastigar v. United States, 406 U.S. 441 (1972), the Supreme Court held that the government need only grant use immunity to compel testimony. The use immunity, however, must extend not only to the testimony made by the witness, but also to all evidence derived therefrom. This scenario most commonly arises in cases related to organized crime."