Fig 1 : the blue volume has a lot of very small spherical balls in it. Balls are under pressure from external system (weight or other), the potential energy is always the same because the blue volume is constant. When I move the blue volume from position 1 to position 2, I can understand that energy needed for move surface S2 is equal to the energy needed to move surface S1. The force on S1 is smaller but the distance to move is bigger than S2. Fig 2 : now, in this case, the shape to move is curve. Like balls are in the blue volume there are forces like black arrows showing. In this drawing I drawn absolute forces from pressure and relative forces from the curve. The S1 and S2 surfaces have forces "up". So it seems the energy needed for move from position 1 to position 2 can recover energy, in this case where I lost energy ?