The Track Start Has No Power
What the track-start position is missing to be a "Powerful" starting position is the counter movement of squatting just before leaping. Explosive vertical jumping typically requires a force to be developed in a time period between 200 and 350 ms. (ms=milliseconds).
Without the sudden counter movement of squatting the resultant explosive movement is not as productive as those which employ a sudden stretch-reflex performance. The swimmer is forced to essentially jump with only one leg, rather than two.
The leaning takes away the immediate effect of the stretching of the elastic muscles. There is no advantage for vertical jumping and usually the back leg is used to gain the height off the starting platform.