I think that only works if you fly direct from Canada to the US. If you fly from one Canadian airport to another, and then to the states (eg. Halifax-Toronto-Orlando, or Calgary-Toronto-Orlando), you can't pre-clear because your first leg of the trip is still considered a domestic flight within the country? If, however, your connecting flights are in the US (e.g. we flew Halifax-Washington-Orlando in Feb.), you can pre-clear (if available at your airport) because you are getting on the plane in Canada and don't touch down again (for the first time) until you are in the US. Your first leg of the trip in that case would be an international flight. Same as if you flew direct (no connections) from Halifax-Orlando, Toronto-Boston...whatever.
If pre-clearance is available, you clear it at the airport where you will board the flight that actually leaves the country. If it isn't available, you have to clear it when you land in the US.