Now obviously, c arrays are just strips of memory allocated to that array, so you can access them using the address of the start of the address and the offset (number of array items to skip). To access them, you sum the address and the offset, then you have the address of the item.

     [~~~~~~~~~~~~~~~~[a~~~~~~~b~~]~~~] Memory
     ^–––––––––––––––––^                Address
                       ^–––––––^        Offset

And now, a[b] is just short hand for the pointer arithmetic going on and you could just use any two ints and access any arbitrary memory address (i assume the compiler enforces that this doesn't happen).

     address[offset] => *(address + offset) => getValueAtAddress(valueIn(address) + valueIn(offset))

    a[b]  => *(a + b) => getValueAtAddress(valueIn(a) + valueIn(b))
    b[a]  => *(b + a) => getValueAtAddress(valueIn(a) + valueIn(b))

Those two are obviously the same so *(a + b) == b[a] == a[b]