but, salt lowers the freezing point of water (that's snow), which would make it melt faster. melt the snow a little, and it packs down. repeat this in layers, and you have a solid jump. throw a little more snow on top to sugarcoat the jump, and you're set.
bangor's got it down.. basically salt lowers the freezing point(as clearly stated ^^) so its melts the snow.. but since you're only salting lightly and it is more than a couple inches on the ground.. the snow melts and refreezes.. it is done all over race camps do it on their courses, and also for freestyle camps in the summer.