How could Lucci's attacks affect Luffy?

[ahill]

Luffy is made of rubber and many ppl in the series showed curiosity the instances Luffy was affected by physical attacks. Yet Lucci's attacks clearly had an affect on him, specially the Six King Pistol (Rokuogan)... how? Maybe the Rokushiki emulates, slightly, Haki?

 

[Future Warrior]

Rokuogan is a shockwave and he was piercing Luffy with his long nails.

 

[sei'taer]

It's a shock wave kind of like the impact dials.