When the fund underperforms its benchmark, this reserve is used to issue refunds at the same rate. As such, half of the underperformance is refunded back to the fund.
The reserve is also used to pay the fund manager a fee. This "draw" is capped at 1/3 of the value of the reserve. The fee is calculated and paid daily, but the rate is annualised.
Furthermore, the draw is capped at an annualised rate of 2.5% of the value of the Standard share class of the fund. This limits the draw when the reserve is large compared to the size of the fund.
The draw will continue as long as there is money in the reserve, regardless of whether the fund is out- or underperforming.
Because the reserve applies to the whole fund, individual investors may benefit from it to different degrees, depending on when they invest or withdraw their money.