They disrupt the surface tension of the water.
Here is a clip from Guiness World Records of the highest shallow dive and explaining how he’s okay. There’s a slow motion clip where he brings his hands together and hold them below where his body is, so his hand hit the water and disrupt the tension so the rest of his body can enter the pool without hurting himself too badly. He also spreads himself out to help him decelerate and pushes forward slightly to displace his downward energy.
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