"He and Jack had implied that Piron might be some sort of threat to the child, but they never believed it themselves -- it was just another turn on the handle when they had put Fiona Wilson through the wringer, trying to squeeze information from her."
(Fiona is Piron's partner.)
Don't know the expression, anyone has any idea?
I don't think this has any meaning outside the context of the story you are reading. They speak of 'putting Fiona through the wringer'. A wringer is an old-fashioned device for squeezing water out of clothes after they have been washed. It is operated by 'turning a handle'.
Of course, the meaning here is purely metaphorical. 'Putting someone through the wringer' is a metaphor for interrogation. When they say 'it was just another turn of the handle' they are saying that it was just part of the interrogation, "it" being the implication that Piron was a threat to the child.