The console opens in a panel at the bottom of the page. It provides a tree of all of the open Realtime documents on the page, including the collaborators on each document and an interactive view of the model.
Edit mode allows you to add, remove, edit or reorder values within the data model. To enter edit mode, select the Edit checkbox in the upper right.
Events mode displays a list of events as they occur. This display includes both local and remote events. To enable events, select the View Events checkbox in the upper right.
In debug mode you can add breakpoints on collaborative objects. When the collaborative object changes, the browser's debugger will pause at the location in the code where the change was made. Note that the immediate pause location will be within the Realtime API code. However, you should be able to locate the location in your code by looking at the callstack.
See it on Github
The debug console is open source on Github. Check out the code that powers it, or provide your own improvements.