为何"break a leg"成了祝福的话?

在World Wide Words里有一篇非常全面的介绍，看过之后就会知道，原来break a leg的典故有很多种说法，没有定论。不过，最常见的解释就是和中文里“好的灵坏的不灵”类似的迷信心理，希望把坏的东西说出来，它就不灵验了。
虽然各种祝好运的场合都可以用break a leg，但被祝福的人需要作出努力（考试、演出、找工作等等都需要hard work）。这么想好了：如果一个人努力了，你祝他/她好运就相当于对努力的肯定，希望努力有回报。如果不需要努力，纯粹只是运气问题（比如中彩票）， break a leg不适用，还是说good luck好。
还有一些与leg有关的词比如说：to shake a leg 就是赶快行动的意思。而To pull one's leg可不是拉后腿的意思而是和别人开玩笑的意思。
Break a leg" is sourced in superstition. It is a wish of good luck, but the words wish just the opposite.
It was once common for people to believe in Sprites. Sprites are actually spirits or ghosts that were believed to enjoy wreaking havoc and causing trouble.
If the Sprites heard you ask for something, they were reputed to try to make the opposite happen. Telling someone to "break a leg" is an attempt to outsmart the Sprites and in fact make something good happen. Sort of a medieval reverse psychology.
Of course it has became a popular wish of luck for theater performers.

塞翁失马，焉知非福。

这是个成语.意思是“祝好运”
这个片语源于古代的一个迷信,当时人们非常相信精灵的存在,而这些精灵最喜欢找人麻烦,制造争端.如果你许了个愿望,他们就会故意让相反的事情发生,于是人们要“祝愿某人好运”时,就会故意说“祝你跌断一条腿”.希望借此骗过哪些精灵,让好的事情发生.
比如：
Break a leg in your test today(祝你今天考试好运气）
I hope your performance in the Golden Concert Hall goes well,break a leg(希望你在金色大厅的演出顺利,