English Explanation of Mishnah Sanhedrin 5:4

They afterward bring in the second witness and examine him.
If their words were found to agree together they begin [to examine the evidence] in favor of acquittal.
If one of the witnesses said, “I have something to argue in favor of his acquittal”, or if one of the disciples said, “I have something to argue in favor of his conviction”, they silence him.
If one of the disciples said, “I have something to argue in favor of his acquittal”, they bring him up and set him among them and he does not come down from there all day. If there is anything of substance in his words they listen to him. Even if the accused said, “I have something to argue in favor of my acquittal”, they listen to him, provided that there is substance to his words.
Mishnah four deals with the court procedure after the testimony has been presented.
After the second witness is examined, the judges begin to examine the evidence. They first examine evidence that might lead to the accused person’s acquittal. The witnesses are not allowed to testify again, even if they know testimony that might lead to acquittal. The disciples, those students of the Sages who sat in rows in front of the judges, were not allowed to speak in favor of conviction. If, however, one of the students was able to raise a point in favor of acquittal, he would be promoted to one of the judge’s seats and from there he could say what he has to say. Finally, the accused himself may testify on his own behalf, provided his claim has some substantial basis.