The MOD command stands for Modulo and returns the remainder from a division. So in my previous statement:
If \y MOD 2=0 Then \y=\y-1
is essentially saying If the remainder of \y divided by 2 is equal to zero then reduce \y by one. It is used here to determine if \y is odd or even, an even number divided by 2 will have a remainder of 0 while an odd number divided by 2 will have a remainder of 1.