The Baki series is well known for its exaggerations of physical abilities. Every fighter in the series is absurdly and, at times, grotesquely muscular. One of the most prominent examples of these physiques comes from the Hanma men's backs.
When a Hanma grows strong enough, they can flex the muscles on their backs in a way that resembles a demon's face. This "Demon Face" (鬼の形相, Oni no Men) grants the user virtually superhuman strength and strikes fear into those who see it. It shows up a few times throughout the Baki franchise and is often a treat to see in action.
How Is the Demon Back Used?
So far, only three members of the Hanma family have been shown manifesting the Demon Face. The first to do it on screen was Yujiro Hamma in the original anime. His son, Baki, would eventually follow in his footsteps. Yuichiro Hanma, Yujiroi's father, also manifested the Face. Jack is the only male Hanma with an ordinary-looking back.
Unfortunately, no one can have a Demon Face in real life. It's not a matter of how toned or muscular they are. It's anatomically impossible because the muscles needed to form such a visage don't exist.
The Demon face is also impossible because of its unique properties. When the user manifests it, their strength, speed, endurance, and durability are all multiplied. It's effectively a shonen anime power-up.
The Demon Face has come to the aid of the Hanmas many times. Yujiro, in particular, loves taking advantage of it. He uses the technique against powerful opponents like Doppo Orochi and Kaku Kaioh. Some see it as unfair, but he sees it as a part of his arsenal of techniques. Because of this, he uses it as often as he pleases.
Baki, on the other hand, is far more reserved about using his genetic gift. He understands the potential immorality behind it, so his gentle nature refutes it. He's only used it a couple of times — once against Jack Hanma and again against his father. He also showed it off in his fight with Biscuit Oliva, though it's unknown whether he maintained it.
Little is known about Yuichiro's use of the Demon Face. It's only implied that he used it in his fight with the U.S. Navy. Beyond that, he's only known as the oldest Hanma ancestor to manifest the technique.
Whenever the Demon Face appears, a brutal beatdown should be expected. This power-up gives the Hanmas an overwhelming advantage in a fight. If they weren't terrifying before showing off this back, they certainly should be afterward.