min() function in iterable index

[OokaydO]

Thank you but I know that the min() function return the most lowest element from a sequence.
My question is about the use of this function in a iterable index and in that form :

alphabet[min(min(line, size -1 -line), min(column, size -1 -column))]

Where min () of min (line, size -1 -line) and min (column, size -1 -column) allows to display in the example (n =4):

a(n2 -1)
a, b(n2 -1), a
a, b, c(n2 -1), b, a
a, b, c, d(n2 -1), c, b, a
a, b, c(n2 -1), b, a
a, b(n2 -1), a
a(n2 -1)

I can't understand the de-incrementing and reversal system of this algorithm.
I don't see how it works and I would like someone to explain it to me.