"abcdef"[3] == 3["abcdef"], but why?

[TEITTINEN at cc.helsinki.fi]

As I was trying to figure out what the cryptic maze program (posted to
net some days ago) does, I ran into a very interesting feature of C. The
program exploiting the fact that the following equation is true

         "abcdef"[3] == 3["abcdef"]   (both equal to 'd')

In fact, not only the values are the same. If the string "abcdef" was
replaced by a pointer to char, the expressions would refer to the same
memory location! This can't be a compiler-dependent feature (or bug)
because the maze program runs correctly on various machines.

Could someone explain to me what a C compiler does when it runs into
expression 3["abcdef"]? 

-- 
EARN: teittinen at finuh                        ! "Studying is the only way
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