Sometimes called a deferred salary or x/y leave, this is an agreement between you and your employer. You agree to defer a portion of your salary over a certain period. For example, in a 4/5 leave, you defer 20% of your salary for four years, and then collect that deferred salary in the fifth year while you're away.
Your employer continues to deduct pension contributions while you're on a deferred salary leave. They report service and salary based on your contract prior to the arrangement. There's no impact to your pension.