Some devices with a DFU runtime allow the user to automatically switch them to I can be programmed mode. This is the right way to do it, and the way the USB consortium wants to do things, but as we all know about vendors is they love to get firmware wrong. For a lot of devices you have to do a little dance, i.e. hold a certain button down when restarting, or connect two pins with a bit of rusty old coat-hanger wire and for that we need the dialog below.
I've hacked up the ColorHug firmware to not support runtime DFU services just for this demo, and it shows what we will ask the user to do. The text on how to prepare the device is specified in the MetaInfo file that's submitted to the LVFS site, and if you need more information on this please just let me know. We're also going to support pictures as well, so if it's unclear we can show a little diagram that makes the text a little more obvious.
Code is very nearly in gnome-software git master.