From David's project page:
The USB OTG cable was needed for the connection to the camera, but the usual ones had a large usb connector on the end, into which you can plug another cable. That was too bulky ( and not very nice looking) for what I needed, so I split apart a couple of regular USB cables and made my own OTG cable by shorting pins 1 and 2 together. Much neater than an off-the-shelf solution.
from David's paragraph above the previous one:
For this initial prototype, I have a DC-DC converter in there to convert from the LiPo’s 3.7 v to 5V, so that I can feed in the power to the ‘external’ power input on the Edison board. The breakout board can take a battery directly, but then the USB port will not work in host mode, which I needed for connecting the DSLR. The DC-DC converter allows me to get around this limitation, and it still all fit into the small box.
His method was to short pins 1 and 2 together on a regular cable to create his custom cable, and he used to DC-DC converter so it would give enough power to power his OTG. Answering both points.