This is from the link above but I still don't understand it!
www.gov.uk/guidance/how-downsizing-selling-or-gifting-a-home-affects-the-additional-inheritance-tax-threshold
*Example
A woman sold her home for £285,000 in October 2018 to go into residential care.
The maximum RNRB in the tax year 2018 to 2019 is £125,000.
She dies in March 2021 with an estate worth £500,000.
She leaves half of her estate to her son and half to her nephew.
The maximum RNRB in the tax year 2020 to 2021 is £175,000.
There’s no entitlement to the transferred RNRB when she dies.
The maximum RNRB at the date of sale was £125,000.
The house was sold for £285,000. Divide this by the figure at step 1, but limit the percentage to 100%.
There’s no home in the estate so the percentage here is 0%.
Take away the percentage at step 3 (0%) from the percentage at step 2 (100%). The percentage at step 2 stays at 100%.
Multiply the maximum RNRB (£175,000) by 100%, so the total lost RNRB is £175,000.
The value of the home at the time of the sale is more than the maximum RNRB at that time so the whole of the RNRB has been lost.
Before she downsized, the woman’s estate could have qualified for the maximum RNRB at that time. So, when she dies, the lost RNRB as a result of the downsizing is £175,000.
The downsizing addition is the lower of the value of other assets that are left to a direct descendant and the lost RNRB.
As she leaves £250,000 of other assets to her son, a downsizing addition of £175,000 is due.
If instead she’d only left £100,000 to her son and the rest of her estate to her nephew, the downsizing addition would be restricted to £100,000.*