I think what you are referring to, is an attempt to keep the noise away from certain areas of London at night between 0400 and 0600, on a weekly basis.
This method uses the skills of Heathrow Approach Controllers for the benefit of Londoners.
Thanks, that sounds logical. There wasn't any mention in the announcement of it being restricted to specific times of day, but I can see that it would be much more difficult to administer from 0600 onwards, notwithstanding the skills of the controllers.
Do you know how many zones are involved, and how many weeks the cycle takes before repeating ?
Given that it offers benefits to the communities under the flightpaths, as you rightly say, I'm surprised the scheme isn't being more widely publicised.