Example timeline(paraphrased from Fluffypony's example in the Monero Missive):
March 15th: 6 month countdown begins
September 15th and later: Hard fork can be activated! but ONLY if 75-80% of the blocks mined in the past week have been mined with the latest version. if not, no fork!
I like this practice. It's proactive, the community knows about it, there is consensus, the fork does not happen until there are enough nodes mining on the new version. It keeps development resilient and allows improvements/changes/features to easily be incorporated without the *panic* and fighting that we see in other crypto.
Look how difficult it is for Bitcoin to change when it needs to.
I miss the part where decentralized means "no changes, ever". If the community decides in the future that a hard fork is not wanted, they can indeed overpower the core team by not using the new version.
This podcast presents a good explanation of the reasoning behind the rolling hard forks:
https://getmonero.org/2016/06/20/monero-missive-for-the-week-of-2016-06-20.html