The Bane Bane no Mi is a Paramecia-type Devil Fruit that allows the user to turn their limbs into springs, making the user a Spring Human (バネ人間,Bane Ningen?). It was eaten by Bellamy.
The power of the fruit was hinted at with the state of the site where Roshio was defeated. The fruit's actual power was revealed when Bellamy and his crew attacked the leaders of the Saruyama Alliance in order to steal Mont Blanc Cricket's gold.
The major strength of the fruit is that it gives its user the ability to transform parts of his body into springs. By applying this to the legs, it grants the user amazing jumping and bouncing power, which can be used to launch himself at incredible speeds, in order to deliver attacks enhanced by a high magnitude of momentum.
Due to the reactive force of bouncing off a solid surface, the user can break apart said surfaces, as the Going Merry was split in half as a result. By applying this ability to other portions of the body, such as the arms, it can release strong, longer-ranged punches. The user can also turn his abdomen into a spring, which grants him more flexibility.
The weakness of this fruit's powers lies within the self-propulsion by turning the legs into springs: the user can only be propelled in a straight direction, making it easy for the opponents to predict the trajectory and counterattack. Also, the user needs something solid to bounce off of, thus the direction is based on the geography of the battlefield; that being said, this also leaves heavy collateral damage depending on how the user decides to bounce. However, such weaknesses do not apply when the power is used to launch a spring-loaded punch or kick. Furthermore, this fruit depends much on the user's physical strength. The fruit's powers would not be effective if the user is physically weak. Other than that, the fruit has the standard Devil Fruit weaknesses.
Prior to the timeskip, the basic concept of Bellamy's usage of this power is to turn his legs into springs and then launch himself as a projectile, with his fist held in front of him to punch an opponent at heightened velocity, and he heavily relied on this power for his combat style. Outside of combat purposes, the fruit's powers have been seen being used by Bellamy to jump to the top of a tower where Luffy was calling out for him.
Spring Snipe (スプリング狙撃,Supuringu Sunaipu?, literally meaning "Spring Sniping"): Turning both his forelegs into springs, Bellamy launches himself directly at his opponent with a punch. This was first seen being used on Shoujou when Bellamy and his crew attacked the Saruyama Alliance leaders. While doing this attack, Bellamy inadvertently also broke the Going Merry in half. In the FUNimation dub it is called the Spring Sniper.
Spring Hopper (スプリング跳人,Supuringu Hoppā?, literally meaning "Spring Recoil Man"): Bellamy's ultimate attack. Using the surrounding geography like the buildings in Jaya's square, he bounces from one spot to the next, increasing in speed with each successive spring, while also disorientating opponents with the continuous sound of springing. He keeps increasing his speed to such a level that he seemingly disappears to spectators. Then he simply has to aim himself fist first at the target and let the accumulated momentum do the rest. This is the technique Bellamy used to defeat Roshio, which caused a good deal of collateral damage on the surfaces he bounced off of, and was first properly seen when he battled against Luffy, the latter of whom possessed enough reflex and insight to keep up with Bellamy's increasing speed and completely nullified the Spring Hopper by punching Bellamy right in the head. After the time-skip, Bellamy combined this with Busoshoku Haki, managing to strike Luffy, who was too restricted by his emotions to counterattack like two years ago.
After the timeskip, Bellamy's style of usage of the power seemingly changed somewhat, such as enforcing his punches by turning his own arm into a spring, as well as evading an enemy by changing one leg for a quick evasive maneuver. He is also able to change both of his legs into one single spring to supplement his acrobatic skills, and utilize his spring-arms as extensions, similar to Luffy's rubbery arms.
Spring Death Knock (スプリング死拳,Supuringu Desunokku?, literally meaning "Spring Death Fist"): An attack similar to Luffy's Gomu Gomu no Pistol, Bellamy turns his arm into a spring and compresses it. He then releases it as a long-ranged and powerful punch. This was first seen used against Tank Lepanto in the Corrida Colosseum.
In the anime, after being forced to defeat Bellamy a second time by Donquixote Doflamingo, Luffy created an attack that simulated his Spring Hopper technique called Gomu Gomu no Spring Bullet as a homage to him.