When performing the Double Boost, Sonic the Hedgehog and the Avatar focus their power into their hands and then high-five each other. The two then do a midair Spin Jump before dashing away side by side, each one performing a Boost that augment the other, thus greatly amplifying their combined speed.
The Double Boost is triggered when a Real-Time Interaction is initiated, in which the player must tap / enough times within a few seconds. If performed correctly, Sonic and the Avatar will boost away like when performing the Boost, but at far greater speeds, while a short version of the song "Fist Bump" replaces the background music. In some cases, "Fist Bump" replaces the background music in its entirety. The Double Boost ends when the Boost Gauge runs out, but it can be refilled by destroying enemies and collecting Rings. If not done correctly, Sonic and the Avatar will stumble for a second before the aforementioned Boost is performed.
The Double Boost is required for the Null Space stage to escape from the titular trap that Sonic and the Avatar were put into by Dr. Eggman as well as the ending of Final Judgment, where the Avatar and Sonic need to escape from the doomed fortress after destroying the Phantom Ruby backup generator. In addition, using the Double Boost is needed to deliver the final blow to both Metal Sonic and the final Infinite fight during their respective battles.
A similar technique called Triple Boost was utilized as the coup de grace against the Death Egg Robot, which was performed by Modern Sonic, Avatar and Classic Sonic.
The Double Boost is known in Japan as the "W. Boost".
During the first Double Boost section in Arsenal Pyramid, if the player runs out of Double Boost (by actively dodging the enemies you're racing towards), the player will continue running at the same speed, just without the boost effect. Additionally, they will not collide into enemies; instead, they will phase past them.