When you blow air inside the funnel, by pressure and density gradient, air will flow upside through the narrow funnel part. When air coming out, it drags some water outside and makes the geyser effect. When you blow harder, the pressure inside the funnel increases even more and, therefore, the sneezed water will be higher than before, i.e., the result is even more spectacular. In the particular case of the real geysers, the pressure originates from the boiling water that is in contact with hot volcanic rocks inside hearth. Try it and learn with Science at Home!