I started with Hardiebacker for the first time today for the wall of a shower, and was greatly impressed by the ease of the score-and-snap method for getting the large pieces I need. However, I also require some thin strips of Hardibacker for a few places (e.g., 3.5 inches wide for the entrance on either side of the curb; 1.5 inches wide in a corner). In those cases there isn't a lot of board to provide much leverage to snap, so how can I make such thin strips reliably?
After scoring, place the sheet on the floor as usual, then cover the thin strip with a stiff board or straight stud as long as the score line. Align one edge of the board with the score. Kneel, or have someone stand, on the board. Pull up on the larger piece.
In other words, score and snap as usual, but trap the thin piece between your work surface and a stiff strong board.
There is another way that doesn't require sawing.
All you need are two 2x8s with a 1/2 inch plywood spacer between them at each end, joined securely with several 3" screws, so that there is a 1/2 inch gap between the two boards.
Lay this flat on the floor. Slip the section you want to cut off between the 2x8s with the scored cut line on top, EXACTLY on the edge of the 2x8s.
Put your full weight on the 2x8s with one leg & step down on the Hardiebacker with the other. If it doesn't snap straight, use another board to apply even pressure the entire length & step down on that.
If you're cutting a narrow strip from a large sheet of Hardibacker, you can lift up on it instead. You may need someone else to lift up on it.
This will create copious amounts of dust, it is extremely important to wear a mask and safety goggles. You should also use a shop vac to minimize dust.
Other answers are also correct -- just to add that an angle grinder with a diamond blade can either cut all the way through or score so heavily that it's easy to snap. (Plus, the grinder will come in handy for tile cutting.)
Can't emphasize enough the danger of this dust, though. Good mask, good dust collection, good eye protection are all necessary.