the Backcut is using the clip edge or secondary edge to cut with. It draws the cut and cutting object inwards. The old backcutters, the Bowie fighters used it to cut someone from the blindside..
Sabers (swords) use the back cut.
Many Filipino arts utilise the back cut!
a simple version can be seen by holding ones right fist, upward thumb raised, then rotate thumb and hand counter clockwise, the action of the thumb represents a simple back cut.