Answer

As is well known, this itself is subject to dispute. But according to the Tosafot mentioned above, it seems to me that the simple explanation is that in a migo of “had he wanted, he could have remained silent,” the person with the migo becomes the one in possession. A migo of “had he wanted, he could have claimed” is evidence used to extract from one in possession. The explanation is that “one in possession” means someone who can say to the other, “I am entitled without any litigation. If you want to conduct a legal proceeding and extract from me, bring proof.” In a migo of “had he wanted, he could have remained silent,” that is exactly the situation. If he had remained silent, he would have prevailed without litigation. Therefore he is considered the one in possession, and the burden of proof rests on the other party. In a migo of “had he wanted, he could have claimed,” he needs to make claims, meaning he does not prevail without proof, and therefore he is not considered the one in possession. The migo is evidence, but the one in possession is the other party. We do not invoke migo to extract money.