A witness shall not be excused from answering any question as to any matter relevant to the matter in issue in any suit or in any civil or criminal proceeding, upon the ground that the answer to such question will criminate, or may tend directly or indirectly to criminate, such witness, or that it will expose, or tend directly or indirectly to expose, such witness to a penalty or forfeiture of any kind:
Provided that no such answer, which a witness shall be compelled to give, shall subject him to any arrest or prosecution, or be proved against him in any criminal proceeding, except a prosecution forgiving false evidence by such answer.
Legal Commentary & Landmark Judgments: Section 137
Section 137 establishes that a witness shall not be excused from answering any question relevant to the matter in issue upon the ground that the answer will criminate them, but provides a statutory immunity that no such answer shall subject them to arrest or prosecution.
When your witness hesitates to answer an incriminating question during cross-examination, request the court to direct the witness to answer under Section 137 BSA; this activates the statutory proviso, granting the witness absolute immunity from prosecution.