"Overclocking key allows switchover between quadruple independent +12V rail mode and high-performance single-rail operation"
Strictly a guess on my part but if you had multiple video cards installed maybe you would use this switch.
Just found this as well.
The design of the Dark Power Pro 10 is based on a single 12V rail with a rather impressive 70Amps. However this 12V rail is splitted into four rails in distribution and all four rails are OCP protected individually. In our opinion and experience, it is quite good that be quiet! decided to split the single rail into four different ones, at least for security reasons, and it can be overridden via the included "overclocking key" switch.
Unfortunately, it appears that the four rails are too heavily OCP protected as even with normal usage, it appears that the device goes into OCP status (for example: a 45A rail providing four PCIe 8-pin connectors and connected to two Radeon HD 6990 cards on them makes the device to go into OCP). We would be quite more happy if the be quiet! provided a bit more information regarding the OCP.
The PSU has four 12V rails, two providing 30A and two providing 45A per rail. The be quiet! Dark Power 10 has a so called "overclocking key" switch which simply disables the individual OCP and turns the PSU into a classic single rail design. It simply groups the four individual 12V rails into a single 12V rail but also provides additional risk, although not that great as it would be on a 1'250W PSU.