In the two-cycle engine, the piston ACTS as a valve, clearing valve ports in the cylinder wall as it nears BDC.

在二行程发动机中，活塞起到阀门的作用，当活塞接近下止点时，使气缸壁上的进排气口打开。

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This, some say, ACTS as a safety valve, allowing citizens to vent their anger directly rather than take to the streets.

有人认为，这就像一个安全阀一样，市民不需要走上大街，就可以把愤怒直接发泄出来。

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With an asynchronous model, the queue itself ACTS as a relief valve.

借助异步模型，队列本身可以充当一个减压阀。

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If bootlegging acts as a safety valve to keep software prices sensible, might that not be such a bad thing after all?

BBC: Why is it 'OK' to nick software?

It acts as a safety-valve for a country that has an unemployment rate of 20% and no welfare provision.

ECONOMIST: Morocco