Luffy actually blocked the tip of Doflamingo's foot[
You must be registered for see links
]
The initial impact of the blow actually looks like it's drawing blood from Luffy's body, indicating his Haki might have been cut; though I think this might just be weird coloring going on, it's worth pointing out.[
You must be registered for see links
]
And here Luffy's body was cut anyways[
You must be registered for see links
]
Luffy's elasticity helped because instead of the strings being able break the structural integrity of his body, his body just deformed under the pressure with the Haki defending against the blunt part of Doflamingo's attack by matching his Haki-enhanced kick.
Plus, if Law can do with his Haki in base what Luffy needs to enter an enhanced transformation to do with his Haki, it just means Law is better with Buso anyways.