ne'emanus of an eid echad for what is b'yado

If while working with taharos someone delcares that the taharos have become tamei, the worker is believed (Gittin 55) and the presumption of taharah is dismissed. Why? Abayei explain that this is based on the idea of "b'yado". Had you asked me, I would have explained that "b'yado" is a type of migo. Since the person working with taharos has the power to contaminate it with tumah, he is believed if he declares that the taharos have in fact become contaminated. But since you didn't ask me, we can learn the better pshat of the Rosh (#13) : ) It makes for a far fetched claim, says the Rosh, to suggest "since he could have contaminated the taharos himself..." when the worker would never do so because of the risk of liability - if caught contaminating taharos, he would have to pay for them. The Rosh therefore suggests that this is a new type of ne'emanus that has nothing to do with migo. The worker is believed based on the principle of ba'alus, i.e. a person has ne'emanus over whatever is in his control.