The new USB-C standard cables are reversible, flippable and the device circuits communicate with each other to regulate the amount of power transferred. I would expect to see some kind of Settings panel that allows you to select which way power is transferred.
From a Yahoo Tech article that has that has more details and animations...
Type-C also offers something called bidirectional charging. Have you
ever had 100 percent battery life on your tablet and 0 percent on your
phone, and wished you could just transfer battery power from the
tablet to the phone? With Type C, you’ll be able to do just that.
You’ll not only be able to charge your laptop, smartphone, or tablet
from the wall, you’ll be able to charge it from other devices, too. So
you could use one laptop to charge another, or a phone to charge a
tablet, or an MP3 player to charge a toaster (if it can get charged
It's brand new so only a few key devices currently have it...
Right now the only devices using Type-C are Google’s new Chromebook
Pixel, Apple’s recently announced MacBook, Nokia’s N1 tablet, and a
few storage devices.