developerPrivate is an undocumented API which allows you to do some crazy things like open devtools programmatically, or launch a chrome app that's under development. The chrome app team uses it in their development environment. The API itself can change at any time and generally apps shouldn't use it. But it's very useful for certain classes of apps.

https://code.google.com/p/chromium/codesearch#chromium/src/chrome/common/extensions/api/developer_private.idl&q=developerPrivate&sq=package:chromium&type=cs&l=5