The shape of the bag should distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater. If the open end reaches the surface, air will escape from the bag and it may re-descend. A partially filled bag will accelerate as it ascends because the air in the bag expands as the pressure reduces on the ascent, following Boyles law, increasing the bag's buoyancy.
It is adviseable to select a lift bag with an appropriate capacity for the task at hand. If the lift bag is grossly oversized a runaway or otherwise out of control ascent may result. Commercially available lifting bags may also incorperate dump valves to allow the operator to control the bags buoyancy during ascent.