How does disk utility work while the system is live? (e.g. partitioning/resizing the OSes partition)

How is it that Disk Utility allows you to resize the partition that OS X is botted from live, while the OS is running? Can there be problems doing this? I would have thought that booting into recovery mode to do these things would be a requirement.

Here's what I mean, Disk Utility will allow me to "Apply" this change while booted in OS X:

I'm not sure whether clicking the "Apply" button is something safe to do while the OS is live. Is it? If so, how/why?