On flexible retirement you can choose whether to take all or some of the pension you have built up. The underpin check is carried out when you flexibly retire, and if your pension increases, a proportion of the increase will be paid to match the proportion of benefits that you are taking that were built up after 31 March 2015. A further underpin check will be carried out when you take the remainder of your pension.
If you flexibly retired before 1 April 2022 and remained a member of the LGPS (NI) you will build up further pension that also has underpin protection. NILGOSC will carry out a further underpin check when you retire.