From April 2014 all members have the option to elect at any time to pay half their normal contribution and build up half of their normal pension. The reduction in contribution would apply from the beginning of the next available pay period.
An employee cannot complete a 50/50 election prior to becoming a member of the main section of the 2014 Scheme. They can then elect to join the 50/50 section, and if they do so before the first payroll is closed they can be brought into the 50/50 section from the first day of their employment. 
The employer contributions are not halved and remain payable in full, with the impact accounted for at the next triennial valuation
Members can elect to move from the main section (and vice versa) as many times as they wish. The member can request a copy of the 50/50 election form from the employer  or can be downloaded from www.yourpension.org.uk and return it to their employer
For further information please see the 50/50 Section membership from 1 April 2014 - Employer Notes
Employee's election to join the 50/50 section
Employee's election to re-join the main section