Here's how it works: You request a code online, by text, or by calling your provider. You're then free to shop around and when you're ready to make the switch, you quote the code. Everything is transferred within one working day (including your number, should you wish to keep it.) There's no overlap — the system ensures your old and new contracts link up perfectly. Ofcom is abolishing "notice period" charges too, so it doesn't matter when you want to cancel (if you have outstanding fees, though, like a part-paid handset, they'll be quoted when you request the code.)
There are numerous benefits. For one, it's possible to cancel your current contract without calling anyone. That means you don't have to wrestle with a salesperson as they try to persuade you to stay. For another, you don't have to worry about PAC codes or a temporary number while you transfer your old digits. Finally, the system should ensure a clean break from your old contract — no more wondering if everything, finally, has been paid off with your previous provider. Such a system will take a while to set up, however, so Ofcom has set a compliance deadline of July 2019.