פירוש על סנהדרין 5:2

בן זכאי – Rabban Yohanan ben Zakkai. And at that time, he was a student who judged in the presence of his teacher; therefore, they call him “Ben Zakkai.”

The more a judge examines the evidence the more he is deserving of praise. Ben Zakkai once checked with regards to the stalks of figs.
What is the difference between inquiries and examinations? With regards to inquiries, if one [of the two witnesses] says “I do not know”, their evidence becomes invalid. But if to one of the examinations one answered, “I do not know”, or even if they both answered, “We do not know”, their evidence remains valid. Yet if they contradict each other, whether during the inquiries or examinations, their evidence becomes invalid.
Mishnah two continues to discuss the inquiries and examinations performed on the witnesses by the judge.
This mishnah discusses the questions regarding the circumstances of the crime itself. Although the mishnah lists seven official “inquiries” that must be asked, with regards to “examinations” the more the judge asks the better able he is to ascertain the truth. Ben Zakkai (who is usually called Rabban Yochanan ben Zakkai) once even checked to see if the witnesses who testified that a murder was committed under a fig tree knew what the stalks of the figs looked like.
The second half of the mishnah discusses cases where one of the witnesses does not know the answer to one of the questions. If the witness could not say where the crime took place or when it took place, both of the witnesses testimony becomes invalid. These are called “inquiries”. However, if one cannot answer with certainty one of the substantive questions regarding the crime, the rest of his testimony is not invalidated. These are called “examinations”. Even if both cannot answer the question, the other parts of their testimony are not necessarily invalidate. In other words, not knowing a detail does not necessarily disqualify all of their testimony. Rather the judges will have to decide when making their decision if there exists enough testimony to convict the accused.
If, however, the two witnesses disagree with regards to a detail, then all of their testimony is invalid.

בעוקצי תאנה – that they were testifying about him that he killed him underneath the fig-tree, and Ben-Zakkai examined, whether its peduncles were thin or thick. The עוקץ /peduncle of fruits is the tail of the fruit where it is attached to the tree.

אמר אחד איני יודע עדותן בטלה – for you cannot further prove them to be false/plotting witnesses with that cross-examination, and all the while that you are not able to fulfill the teaching of false witnesses with one of the witnesses, their testimony is invalid, and even if they are one hundred [witnesses], for witnesses are not made false until all of them are proved to be false/plotting [witnesses].

בדיקות – even if all of them said that we do not know, the command of false/plotting witnesses is worthy to be upheld, since false witnesses is not dependent upon anything other than to say” “You wiere with us at that particular hour in a particular place.”

עדותן בטלה – all of their testimony is invalid [that is in the Gemara] and he and they are exempt.