Despite reports from players believing the button is not yet implemented on the PTR, it actually does work. As seen in the video, the button allows you to navigate from a meta achievement to one of its requirements and back again, which is very helpful to see the exact criteria for Coiled to Strike without losing your place and having to find Ula'tek Uncoiled again.
However, what the button doesn't do is take you from any achievement to the previously viewed one. The functionality only appears available for meta-achievements... which is still very useful when jumping between categories, just limited. Perhaps at some point, a more robust browsing history can be implemented in order to allow players to navigate back and forth freely, but this seems to be it for now.
It even works across multiple levels, so you can navigate back through several nestled meta-achievements.
