Today when we witness a crime, we bear testimony of it, and if the alleged criminal is convicted, the justice system carries out the penalty, but in ancient Israel, it did not work that way.
The witnesses also had to be the first to carry out the justice.
The Lord told Israel,
“The hands of the witnesses shall be the first against him to put him to death, and afterward the hands of all the people. So you shall put away the evil from among you” (Deuteronomy 17:7).
Could the witness in a good conscience do these things? If so, that added creditability to his testimony, and it would help to discourage false testimony.
The Lord still expects us to bear true testimony, for we shall give an account to Him for every word that we have spoken (Matthew 12:36).