* "Lock-On" Technology: The cartridge had a special slot that could accept other Genesis cartridges. When a compatible game was inserted, Sonic & Knuckles would analyze the game's data and modify its gameplay.
* Specific Games: This feature worked only with a few select Genesis games, most notably:
* Sonic the Hedgehog 2: This combination unlocked the legendary "Knuckles the Echidna" as a playable character.
* Sonic the Hedgehog 3: This combination offered a more complete version of the game, including all of Knuckles' levels and a modified ending.
* Sonic & Knuckles: If you inserted another copy of Sonic & Knuckles, you unlocked a special "hidden" mode called "Sonic & Knuckles 3D Blast" which was a different style of game featuring levels with a 3D perspective.
* Incompatible Games: If you inserted a game that wasn't designed to be "locked-on," it would either fail to load, or the game would start normally, but Sonic & Knuckles wouldn't have any effect on it.
Why this was special: This technology was a unique way to offer expanded content and new gameplay experiences. It was a clever way to encourage players to purchase multiple games and explore different ways to play.