This is a question that I regularly get asked on and off shoots. To make things simpler I thought I should post a quick description of what it is, why the option is there and the difference it can make.
The option to automatically use OpenCL or not was introduced to Capture 6.0. In version 7.0 this was made slightly more complicated as the option is now available separately for image display and file processing.
To start with OpenCL stands for Open Computing Language and in terms of Capture used to allow programs to tap into the processing power of graphical processor unit (GPU) on the graphics card of your computer.
Current graphics cards are actually incredibly powerful processors on their own. The major manufacturers, AMD and Nvidia, have pushed the technology hard to be able to render very fast, detailed graphics for gamers. This has led to graphics processors with multiple cores and very high clock speeds (MHz/GHz) targeted at rendering detailed textures to screen quickly. This power goes unused most of the time as the computers main central processor unit (CPU) is normally used to process files in Capture One.
The theory is that with the Auto setting in Capture One, the software is able to access this extra processing power when it is rendering previews or processing out files to TIFF or JPEG formats.
The reality is that unless you are using an Apple computer with a recent graphics card and the operating system is at minimum Snow Leopard (10.6) or above, the technology is not supported.
As a general troubleshooting tip, if there is any doubt regarding the ability of your mac to support this feature, then turn it off. Also if Capture running slowly or at all jittery, I would recommend it as things to check and switch off.
If you want to go into further detail about Phase One's own advice on the subject, then go here. And for a detailed list of the Apple computers that will definitely not support this feature you can always check this page.