If you think of it sort of as a number line, the name "Christopher" would occupy spots 1 thru 11 of the line. The expression is askiing you to look at spots -6 thru 5. This is 12 spots because of the number zero. The first 7 spots (-6 thru 0) we're asked to look at don't correspond to any characters in the name and don't return any characters. But the last 5 spots correspond to spots 1 thru 5 of the number line and those spots are occupied by the characters "Chris".

This is perhaps a clumsy example, but it seems to work.