It is not that Asrock and AMD got together to come up with this feature. AMD would much rather you spend more money on one of its more expensive multi-core cpus. It is more a marketing gimmick by Asrock, and by using the word "gimmick," I am not saying that this is not an useless feature. For a company that sells motherboards, offering a board with more versatility as well as more bells and whistles at the same price level as a competitor's less versatile motherboard will usually mean that the company offering the more versatile board will appear more attractive to the consumer; therefore, attracting said consumer to spend his or her dollars with them instead of their competitor. Asrock isn't the only company to offer a mobo that has a cpu core unlocking feature either, and this feature does work; however, I recommend you read this article before you entertain the idea of doing so: Unlocking CPU Cores
It isn't that the core unlocking feature is creating a new core on the AMD CPU, but actually unlocking a core that was disabled because a core or cores did not pass a test for some apparant reason (Toms Hardware.com).
Yes, it is possible to unlock disabled cores, but it might not be stable. This is very much akin to overclocking the CPU or a GPU. It can be done, but it might or might not be stable. I have never attempted core unlocking; however, I used to be heavy into overclocking. I have a feeling what holds true for having a system that supports successful overclocking holds true for unlocking cores: finding hardware components that will remain stable with the core unlock.
For additional reading, a article I found helpful to get me up to speed on multi-core processors last year.
Hope that helps.