What is the difference between flat breaststroke and wave breaststroke and which one is more energy efficient

On the left is the flat breaststroke, and on the right is the wave breaststroke. In each action cycle, the body position of flat breaststroke is very close to the horizontal position, the hips are always close to the water surface, and the inhalation action is done by lifting the head, so it does not destroy the horizontal position of the torso. In the wave breaststroke, however, the head and shoulders rush out of the water when inhaling, and the hips sink deeper when closing the legs.

? The biggest difference between these two swimming styles is shown in the diagram above, c and d. When an athlete using the flat breaststroke technique closes his legs, his body remains in a horizontal position. At this point his shoulders are still underwater and his hips are near the surface. In contrast to the flat breaststroke, the wave breaststroke swimmer's shoulders are out of the water, his hips are lower, and his body position is tilted. During the rest of the movement phase, there is no great difference in body posture between the two strokes. During the thrust phase of the arm action (Figures, a and b), athletes using both stroke techniques maintained a good horizontal body posture and good streamlining. During the thrust phase of the leg action (Fig., e), the body maintains the same posture as described above. At this point the only difference between these two swimming styles is shown in Fig. 1. The competitor using the wave breaststroke has a slight hip lift at the end of the stroking action.?