Explaining Opt-in Transparency in our texts would have definitely taken up wayyyyyyyyyyyyy too much space, not because it's complex, but because texts have a way of getting HUGE. Anyway, I figured I'd chuck this together quickly and just link to it in our text exchange :)
Opt-in transparency is a technique for managing asynchronous communications so that people can kind-of manage their own level of participation in an internal decision, project, task, or whatever.
By using certain tools, which you may already be using, you can document the context in which designs and decisions are made. Moreover, if this documentation is made widely accessible within an organisation, people with an interest in the design or decision, can view the contextual information surrounding it.
tbh, that's really it; however, it's probably worth adding that certain ways of doing opt-in transparency - or certain tools you might use to implement it, allow people to participate more actively, in the process by which the designs and decisions are made, rather than simply understand the context behind them.
Zach Holman wrote a lil article about this in 2015. In this article, Zach mentions that Opt-in transparency was created to allow a company to be "fully open as an organization, but not force that openness upon everyone by default".