The most significant secret of Sonic & Knuckles is its revolutionary 'Lock-On Technology'. By physically attaching other Genesis cartridges to the Sonic & Knuckles cartridge, you unlock new gameplay experiences.
- Sonic the Hedgehog 3: Locking on Sonic the Hedgehog 3 creates Sonic 3 & Knuckles, the definitive version of the game. This combines both games into one massive adventure, allowing you to play as Sonic, Tails, or Knuckles through all 14 zones. It also introduces the Super Emeralds, which unlock Hyper Sonic and Hyper Knuckles, and Super Tails. This is highly recommended for the complete experience.
- Sonic the Hedgehog 2: Locking on Sonic the Hedgehog 2 allows you to play as Knuckles in all of Sonic 2's zones. Knuckles' unique abilities open up entirely new pathways and secrets in these classic levels, offering a fresh perspective on a familiar game.
- Sonic the Hedgehog 1: Locking on Sonic the Hedgehog 1 unlocks the 'Blue Sphere' game, a randomized version of the Special Stages from Sonic & Knuckles. You can play an infinite number of these stages, making it a fun diversion and a way to practice your sphere-collecting skills.
- Other Genesis Games: Locking on any other Genesis cartridge will also unlock the 'Blue Sphere' game, but with a unique, randomized seed based on the cartridge's ROM data. This means every other game offers a slightly different set of Blue Sphere stages.