Understanding git.git's branch policy

Radding the maintain-git.txt document¹, there are a few points that I'm having trouble decoding. Under "The Policy", it notes

,----
| The tips of 'master', 'maint' and 'next' branches will always
| fast-forward, to allow people to build their own customization on top
| of them.
`----

I understand that a "fast-forward merge" means that one's current HEAD commit is an ancestor of the evolved branch's head, so that the HEAD pointer can move forward to "catch up" without needing to combine disparate content.

How does this relate to the prescribed use of the "master", "maint", and "next" branches? What operations or patterns does it constrain against?

Footnotes: 
¹ http://kernel.org/pub/software/scm/git/docs/howto/maintain-git.txt