hi karri,
I am not sure , but i think buf refers to first element , (buf + 1) refers to second element and [5] means shift to 5 elements ahead. Then it will go to 7th element.
If you have gcc or cl you can create assembly code and see how its working.
For creating assembly code type gcc -S filename.c or for cl type cl /Fa filename.c

kaustubh.