Halakhah for Sanhedrin 5:8

And besides all of this that we mentioned that are called investigations and inquiries - which are the essence of the testimony and with them the accused is made guilty or exempted - the court would engage in much checking of the witnesses in other matters that are not very essential to the testimony. And since they are not essential, they, may their memory be blessed, called them, checks. And they said about them (Mishnah Sanhedrin 5:2), "All who increase the checking are praiseworthy." And what is that which is called checks? For example, "What was the killed or the killer wearing?" And likewise, we ask him, "Was the earth on the land that he was killed white or red," and similar to these matters. And they, may their memory be blessed, said (Mishnah Sanhedrin 5:2) that with the investigations, if one witness was precise in his testimony and the second one said, "I do not know," their testimony is nullified; but with the checks, even if both of them said, "We do not know," their testimony [still] stands - and all the more so, if only one says, "I do not know." And about what are these words speaking? When they did not contradict one another. But if they contradicted one another even in the checks, their testimony is nullified.