Chakra collided , that's all
It had nothing to do with narutos strength but the force of the two cloaks collided , like Naruto and sasuke chidori and rasengan
During the battle with Kaguya Sakura landed a surprise attack on Kaguya but only struck her head and damaged her horns which were delicate so they broke easily.
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Her punch didn't even push Kaguya back it only forced her head downwards
But when Naruto punched Kaguya through a different kind of surprise attack his punch managed to knock her away a considerable distance.
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Not only did this punch push her back, it also kept her stunned long enough for Naruto and Sasuke to almost seal her
Chakra collided , that's all
It had nothing to do with narutos strength but the force of the two cloaks collided , like Naruto and sasuke chidori and rasengan