"Back feeding" a breaker is simply when you delover power to the screw terminals, and power exits the breaker at the buss connections. That is, current flow is in the reverse of the usual direction.
This method is commonly used in "all in one" panels, where the 'back fed' breaker is the first, and only, means of service disconnect.
It's also used to provide a local disconnect to a 'sub panel,' or when the panel lacks any sort of lugs on the busses.
