The Accel Synchro Monsters are a new type of Synchro Monsters who can only be Summoned using a new method called Accel Synchro Summon. This new type of monsters debuted on Yu-Gi-Oh! 5D's - Episode 074, who are used by a duelist known at that time as Dark Glass, who is now revealed to be Bruno from Team 5d's.
However, summoning an Accel Synchro Monster uses multiple Synchro monsters as Synchro Material Monsters. Performing an Accel Synchro Summon is treated as Spell Speed 2, as Dark Glass is able to chain his Accel Synchro Summon to Yusei's activation of a card effect (on Yusei's turn). This, coupled with the individual effects of the Accel Synchro monsters, provide a defense against effects that target Synchro Monsters, such as but not limited to the Infinity (archetype) monsters, leading Yusei and the rest of Team 5D's to pursue the secrets behind Accel Synchro Summoning during the events leading up to, and during, the World Riding Duel Grand Prix.
Tech Genus Blade Gunner MAXX-10000 is the only Accel Synchro monster shown so far. Shooting Star Dragon has been revealed, but not yet shown in card form, and the Accel Synchro counterpart to Red Dragon Archfiend appears in the 4th opening.