This was the maiden flight of my 3D hacks to the Arducopter 2.9.1b code. So while my piloting sucked, I was pretty impressed that it actually worked!
The ESCs are Afro 30s with SimonK configured with COMP_PWM=1, MOTOR_BRAKE=1, BRAKE_SPEED=8 and of course RC_PULS_REVERSE=1 to allow reversing.
The props are actually two sets of carbon fiber props stacked on top of each other with one set upside down - another hack. This is far from ideal, but all I had on hand at the time. Normal props are far too inefficient in reverse and I actually don't have any I can flatten out (yet). I can't wait until 10" 3D props are common!
The code needed quite an overhaul to simply support negative throttle. On top of that, I added some delays when switching direction to make sure the motors stayed in sync when reversing direction. The final cherry was to have the acro mode self leveling mix switch the roll target to upside down when the throttle reversed. What you kind of see in the video is me just reversing thrust and letting the computer flip it over for the inverted hover.