Sanji is a first-rate cook of the sea who joined the Straw Hat Pirates on Baratie. Along with being an excellent cook, he's also one of the strongest fighters that Luffy has. Sanji is considered to be one of the strongest pirates in the entire world, and his bounty of 1.032 billion berries is there to show for it.
He is extremely powerful and he has always relied on his incredible Black Leg Style in combat. He empowers the style with the techniques such as Diable Jambe and now, he has taken this to a new level with Ifrit Jambe. Sanji awakened this power recently in the Wano Country arc and, needless to say, it makes him a fearsome pirate, capable of taking on even the toughest of enemies.
Sanji's Genetic Mutations
Being a member of the Vinsmoke Family, Sanji was experimented upon before birth. His father, Judge, wanted to make his children the ultimate war machines and use them to conquer the North Blue once again. To do this, he used whatever scientific knowledge he had gained over the years and stripped his children of their emotions. His wife, Sora, vehemently argued against him and ended up taking medication to prevent them from becoming emotionless killing machines. Sadly, the drug only worked on Sanji, and the others were turned into monsters. For the vast majority of his life, Sanji had these abilities latent within him, however, at some point, even before meeting Luffy, these powers had actually awakened.
Sanji has a resistance to fire and this has actually been shown in the Baratie arc as well, where he fought Pearl, a man set on fire, effortlessly, without so much as receiving a single burn. Recently, in One Piece Volume 105 SBS, Oda mentioned that Sanji's resistance to fire comes from the genetic augmentation that he underwent. This resistance to fire later built up and he ended up manifesting as Diable Jambe, which is the technique he uses to light his legs on fire.
With Diable Jambe, Sanji can increase both the speed and the weight of his kicks and, thus, gain greater firepower. These kicks are powerful enough to burn through his enemies and make him exceptionally powerful. While fire resistance was the only genetic augmentation that Sanji manifested early on, things changed as the story progressed. After putting on the Raid Suit three times in Wano, Sanji's body started to change. His latent powers that had been suppressed by the drug that Sora took started to awaken and he gained access to all the abilities that his siblings possess, except for having his emotions taken from him.
When these powers manifest in Sanji, his eyebrows flip directions. Just like the Germa siblings, they form a 66 pattern, signifying that Sanji is utilizing the scientific powers of the Germa. Once Sanji awakened these powers, he ended up gaining an extremely tough exoskeleton, an advanced recovery rate, more muscle mass, and a great boost to his strength, speed, as well as all his other physical attributes. When all this was combined together, Sanji's resistance to flames increased even further. This meant that he could now support even hotter flames on his legs. When he combined these powers with the carefully refined Color of Armament that he had developed over the years, Sanji gave birth to the flames of Ifrit Jambe.
The Strength Of Ifrit Jambe
Ifrit Jambe is a higher grade of flames used by Sanji. These flames burn much hotter and this is signified by their blue color. Blue flames are known to be the hottest and Sanji being able to produce these flames means that his body is strong enough to handle not being burned by them. It wasn't possible for him to produce these flames earlier because his heat resistance was not infinite. Sanji could only handle as much as his strength allowed him to. Now that he has gained all these incredible buffs, he can support even hotter flames which are blue, meaning they burn the hottest.
In fact, the core of Ifrit Jambe seems to be covered in a plasma-like substance, created by the ionization of the gas. Furthermore, these flames are supported by lightning as well. Essentially, Ifrit Jambe allows Sanji to make use of the blue flames that burn the hottest in One Piece and produce lightning as well. The power of these flames was such that just a single combo was enough to take down Queen, a powerful pirate with a 1.32 billion berry bounty. His entire body was charred from the inside out and Sanji was able to dismember him without much difficulty. The power of Ifrit Jambe will continue to grow in the future of the story and, as hinted by Oda, the scientific modifications of his body have not stopped just yet. Sanji is going to grow stronger and the power of Ifrit Jambe might even be taken to the next level when that happens.