Hydrostatic pressure only occurs due to weight and the pressure due to that weight. For example, you have a U-tube, both sides equally filled with an semipermeable membrane in between. You pour water into one side, and it'll be very temporarilly higher, but due to hydrostatic pressure (gravity basically) the two sides will even out. Now if that same semipermeable membrane isn't permeable to Na+ or Cl- ions, and you add NaCl to one side of the U-tube, water will shift to that side.