A liverpool bit is hung in a fixed position on the cheek pieces. The reins can be placed on the ring in a snaffle position or on the holes below in the shanks. In the snaffle position it is less severe but gives you more "cranking" power than a snaffle as your rein action creates pressure on the poll. The lower you place the rein the more leverage you create. For a horse that is strong out cross country you may find you need this extra leverage.
I would try it on the snaffle rings first as you will still have extra leverage than usual and then if that isn't effective enough move it first to the top hole in the shank and work your way down till you and she come to an understanding.
With a leverage bit the slightest pressure from your hands is greatly increased.