The piston travels down on the intake stroke, up on the compression stroke, down on the power stroke and up on the exhaust stroke.

进气行程，活塞下行；压缩行程，活塞上行；做功行程，活塞下行；排气行程，活塞上行。

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With the down stroke of a wing during flight, the body moves up, but the subsequent upstroke can counter that force.

用飞行翼的上下行程，身体向上移动，但随后的冲程可以对抗这种力量。

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During the breaststroke underwater pullout, the arms pull DOWN during the power phase, and then arms recover back UP to start the first stroke.

蛙泳水下蹬壁滑行时，手臂在蹬壁出发后要向下拨水，然后手臂恢复开始第一次蛙泳手划水。

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Conventional four-stroke engines produce power by moving a piston up and down using rapid explosions of fuel and air in a cylindrical chamber.

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