Shevuot (Oaths) 30 - The Oath of Testimony
The oath of testimony concerns witnesses who knew testimony that a litigant needs in order to win a monetary case. If the litigant asks them to testify, and they falsely swear that they do not know the testimony, they are liable to bring a "variable" offering when they admit that they lied. If the litigant has other means to win the case, or he has other witnesses, the liability for false oath does not apply.
In addition to making a personal oath, one can answer "Amen" to someone saying "I adjure you," and this also becomes an oath, as if he said it himself. In the case of the oath of testimony, if one repeats in court that he does not know the testimony, after being adjured, this also becomes an oath.
Art: Louis Charles Auguste Couder - The Court Oath