A more thorough explanation:

Jencks material refers to a written or recorded statement made by a prosecution witness before a trial. If a criminal defendant files a motion after the witness has testified, they are entitled to have access to this statement to prepare for cross-examination. The defense can use this statement to challenge the credibility of the witness. This right is protected by the Jencks Act, which is a federal law.

During a trial, a witness testifies that they saw the defendant commit the crime. The defense attorney files a motion to obtain the witness's written statement made before the trial. This statement is considered Jencks material and can be used to challenge the witness's testimony.

Reverse Jencks material refers to a written or recorded statement made by a defense witness before a trial. The prosecutor is entitled to have access to this statement to prepare for cross-examination. This right is protected by the federal law and can be obtained during pretrial discovery.

During a trial, a defense witness testifies that they saw someone else commit the crime. The prosecutor files a motion to obtain the witness's written statement made before the trial. This statement is considered reverse Jencks material and can be used to challenge the witness's testimony.