This is what I have learnt today.

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Actually, rubber-stamp has nothing to do with 夸夸其谈的. It refers to a situation when a body with strong theoretical powers uses that power on the order of someone else.

For example, the National People's Congress (人大) in theory makes the law in China, but in practice it just rubber-stamps whatever the Communist Party Central Committee (党中央) and Politburo (政治局) says.