The reason it works is that maturity is on the year anniversary. Using Nationwide's regular saver as an example. Open on Jan 28th, add £200 on opening then another £200 in a couple days on the first of the month, and each first of the month. Come next January 1st you'll have £2600 in the account, which will continue to gain interest until the 28th.
If you open on Feb 1st, deposit £200 and each month on the first, on January 1st you'll have £2400 which will be your final balance until maturity on the 1st Feb 2024.
Essentially you get nearly a year's extra interest on £200 which is about £15 on the 8% account above.
This works for Nationwide, Skipton and, I think, Gatehouse.