This is taken from so much 'cop' series on TV. It has nothing to do directly with parenting.
You have a subject to be interrogated; two cops are involved; the first one adopts a soft approach and then, not succeeding, the second cop adopts the hard approach, and so on.
Perhaps this has been adopted in some parenting advice so that each parent takes on the hard role, instead of one parent always being hard and the other always soft.
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