_THEORY ON CONNECTION BETWEEN LUFFYS VULCANIZATION AND SANJIS DIABLE JAMBE
CoA haki is an invisible suit of armor that covers ones body .We all know that.However Luffy appears to be able to use it to vulcanize his body parts.Now thats confusing.This is because Rubber only vulcanizes in the presence of Heat,Pressure and a suitable agent.How can a invisible suit of armor provide this conditions?
To answer the this I checked through the manga for clues and noticed that the size of ones CoA haki could be manipulated.For example The Admirals expanded the size of their CoA haki to protect a Platform.
If it can be expanded around an object,then it can also be contracted around an object.Logic should tell you that contractin/compressing a suit of armor around an object would exert heat and presssure on that object.
So by Contracting CoA haki around body part/s luffy supplies the necessary heat and pressure for vulcanization.All thats left is a suitable agent and it seems that the CoA haki itself works a vulcanizing agent.Bottonline is that contracting Coa round Luffys body parts results in vulcanization.
It also dawned on me that if CoA haki can be used to create heat and pressure then it could explain how on earth Sanji is able to trigger Diable Jambe.
Pretimeskip Sanji would heat up his leg during high speed rotation due to air friction.
Post timeskip its unclear how sanji heats up his leg.All he does is say "Diable Jambe"and his leg heats up instantly .So the question is how is he doing it?
My answer is CoA haki.He contracts his CoA around his leg and the heat and pressure produced initiates Diable Jambe.It was already shown pretimeskip that Sanji is highly heat resistant and post timeskip proved that Sanji is highly pressure resistant.Thats why Contracting CoA around his leg doesnt hurt him.
The advantage of using CoA haki instead of high speed rotation to trigger Diable Jambe is thatDiable Jambe activates in an instant as opposed to taking time to warm up.Its also less risky
**Note:**I believe that without using Diable Jambe or vulcanization Sanji and Luffy 's haki attacks/defence would be very weak,almost useless.They are after all beginners so it makes sense they have weak CoA.Their Coa will get better as the series progresses.So each time Luffy and Sanji want to use a CoA attack or defense theyll use Vulcanization and Diable Jambe respectivley._