Start with an existing BSD or CDD licensed codebase, say
FreeBSD or
OpenSolaris (note that not all technology used as part of OpenSolaris is open sourced under the CDDL, but much of it is). As a hybrid-source vendor, you start developing new closed-source sections for this codebase, perhaps 802.11n support or a better desktop environment. When you're ready, you release this new operating system, hybridOS, on Jan 1st, 2011, call it HybridOS version 11.1. 50-80% of the source code is always kept open; the remaining parts are closed, including the new features you've added. You contract with your customers that you will release all source for the closed parts of 11.1 to them within two years, on Jan 1st, 2013 at the latest. There is nothing stopping you from releasing that source earlier if you'd like, 2013 is merely the deadline.
You continue development and come out with a new version on Jan 1st, 2012, call it HybridOS 12.1. This new version supports UWB and has a rewritten bluetooth stack, both of which are kept closed. Skip ahead another year to Jan 1st, 2013 and the release of HybridOS 13.1, which now supports touch interfaces and has a rewritten kernel scheduler. You now have to release all the source that was used to build HybridOS 11.1, ie the source for the original 802.11n support and better desktop environment, ideally under the same open source license that you started with, the BSD license or CDDL. You don't have to release any source that was used in subsequent builds like 12.1, such as the UWB or touch interface patches, at least not until 2014. You don't have to release any bugfixes or security patches that were integrated into later builds after 11.1 came out, only exactly the source code that was used to build version 11.1. Eventually all the closed source sections are open sourced, but not until two years have passed since they were included in a build.