I found a video of some trikers who start out going backwards, but spin around to moving forwards (watch just the first 10 seconds). As best as I know, my trike can only go forwards. How is such a trick achieved?
They're back pedaling as they go, this lets the drive train move backwards.
You'll often see BMXers doing the same, you see more in the first part of this video.
Of course your trike can go backwards. However the pawls in the freehub will engage and the wheel will drive your pedals backwards.
So you can't pedal it backwards... they're just using their feet to allow the cranks to rotate backward.
The trike stays mostly upright because its three wheels - this would be near impossible to pull off on a bicycle.
Also they're using weight transference to unload a wheel, and being on a cold shadowed and icy corner helps to lose traction.