But if you stop moving forward

但当你不再向前运动时
that pressure drop disappears, you go down.

压强差会消失你就掉下去了
Forward motion is required for an airplane to stay up in the air,

对于飞机来说要留在空中就必须向前运动
for a bird,

对于鸟类
most birds that stay up in the air.

对于绝大多数能在空中飞行的鸟来说
Hummingbirds can hover which is a remarkable physical achievement,

蜂鸟的悬停能力是很了不起的
and they do that because they can actually

它们能够悬停是因为
turn their wings in both directions.

它们实际上能向两个方向扇翅膀
They can turn them upside down and that's shown in this diagram here,

它们能把翅膀翻转过来如图所示
that they can make the wing go backward and forward,

因此它们能使翅膀向前扇也能使翅膀向后扇
and turn it right side up and upside down,

可以把翅膀翻转上来再翻转下去
so they can create a motion

因此造就了一种运动