How does disk utility work while the system is live? (e.g. partitioning/resizing the OSes partition)
Although I don't know what specific property of HFS+ allows a user to modify the current system partition while in use; my expectation would be that operations to blocks of storage that the filesystem is not currently utilizing are allowed.

Now if you tried to shrink the system partition below the current size of your system installation (the blue shaded area) disk utility would present an error.

EDIT

Just wanted to add some additional information that I came across. If you refer to the diskutil(8) man page under resizeVolume it states:


  Non-destructively resize a volume. You may increase or decrease its
  size.
  
  A size of limits will print the range of valid values for the target
  partition, taking into account current file system and partition map
  conditions such as files in use and other (immovable) partitions
  following the target.


This seems to confirm my thinking above.