Mishnah 3. In the previous mishnah we established the principle that when a person volunteers information that stands against him, and expressly qualifies it in a way that removes the harm from himself - since he himself brought the damaging information forward, we believe him together with his qualification. Our mishnah presents another example of this principle.
Background: Authenticating Documents:
Someone who brings a document to court - for example, a loan document stating that Reuven owes money - must authenticate the signatures on it, for otherwise it could be claimed that the signatures are forged. But once we have something that supports and testifies that the signatures on the document are indeed those of the witnesses who witnessed that loan, we rule that there is no forgery here, the loan document is valid, and money may be collected on its basis.
The Words of the Mishnah:
Witnesses who are signed on a loan document, who come and authenticate their signatures - "kesav yadeinu hu zeh" - this is our handwriting - and admit that they were witnesses at that event, but who add a qualification and claim that the document is invalid for a different reason:
"anusim hayinu" - we were coerced into signing the document, and we were not genuine witnesses.
"ketanim hayinu" - at the time of signing we were minors and not fit to sign.
"pesulei edus hayinu" - we were relatives, and disqualified from testifying at that time.
These witnesses are believed. True, they testify that there is no forgery here, but they qualify the authentication and reveal that the document is void for another reason. Their credibility stems from the fact that without their words the debt could not have been collected at all - this is a migo: they themselves are the ones saying that the loan document ought to be valid and is not forged, and they themselves are the ones revealing that it carries a flaw for another reason.
When the Signatures Are Authenticated Not Through Them:
Outside witnesses - other witnesses who confirm that these are indeed the signatures of those signed on the document.
Another authenticated document - a document on which their signatures have already been authenticated, and we compare the signatures and determine that they are identical.
In both of these cases we do not need the testimony of the signatories in order to authenticate their signatures, and therefore they are not believed to claim that they were coerced, minors or relatives. Their claim is not a valid claim, since there is no migo here: as long as we need their help to authenticate the signatures, it is in their power to make the authentication conditional and say that the document is void in any case; but once we have outside authentication, their authentication is not needed, and consequently the disqualification they voice is not valid either.
In summary: The signature is considered valid on the strength of the outside testimony - other witnesses or an authenticated document - and the loan may be collected with this document. When the authentication of the document depends on the witnesses themselves, they are believed even in their qualification by force of the migo; and when the document is authenticated from another source, the migo falls away and their claim falls away with it.