Phased retirement which was also known as 'staggered vesting', allowed the purchase of a pension to be phased allowing flexibility when considering retirement.
Phased retirement worked by splitting the pension into many segments each of which could be encashed separately. The pension income is then composed of a combination of tax free cash and annuity from the individual segments. The remainder of the fund remains invested and may benefit from any market growth in its underlying investments.
From April 2015 there is now much greater freedom to choose how you use your pension fund and the rules regarding phased retirement are now largely irrelevant as there is so much flexibility in how you withdraw money from your pension. See New Rules About Pensions