It’s quite simple, when you get your head round it.
Say you owe £900 for 2020-21. You pay £900 by 31 Jan 2022, and you’re done.
But say you owe £1,000. You then have to pay that £1,000 by 31 Jan 22 plus another £500 (half of the tax owed) on account for 2021-22. Then you have another £500 to pay by 31 July 22.
If you then owe £1,100 for 2021-22, you would pay the balance owing of £100 by 31Jan 23, plus your first payment on account for 2022-23 of £550. And then a second payment on account of £550 by 31 July 23.
And so on…
If you know you have had a year high in repairs and you know your tax will be less, you can reduce the payments on account (via your online account), but if you reduce them by too much, they will charge interest (2.6% at the moment from memory).
But if you know you will owe more, you still only have to pay the calculated payment.