Braccio board not powering Yun Rev 2

I've got a Braccio shield for the Braccio arm. I set the voltage switch to "Vin", and indeed I can see that it is putting out 5V when I plug the Braccio shield in, and it clearly is providing 5V to Vin on a Yun Rev 2, but the Yun doesn't start (no LEDs on).

Is there something I need to do to Yun Rev 2 to make it accept voltage from Vin pin?

-Thomas