If you take flexible retirement, you can choose whether to take some, all or none of the pension you built up after 31 March 2015. If the underpin check means that your pension increases, a part of the increase will be paid. The proportion will match the proportion of the benefits built up after 31 March 2015 that you choose to take. A further underpin check will be done when you take the rest of your pension.
If you took flexible retirement before 1 April 2022 and did not opt out of the LGPS, you will have built up further pension that is protected by the underpin after your flexible retirement. Your pension fund will do a further underpin check when you fully retire.
You can read more about flexible retirement on the Taking your pension page.