Lock-On Technology
From Sega Retro
A special feature found on Sonic & Knuckles for the Sega Genesis that allowed a player to connect an older Sonic game, such as Sonic the Hedgehog 3, to the cartridge for extended gameplay. Lock-On Technology works by using special circuitry that combines multiple ROM chips into a single address space. The result is that the console thinks that one big cartridge is plugged in.
Lock-On Technology also enables a special 256K ROM chip when Sonic the Hedgehog 2 is connected. This extra ROM chip is required for proper gameplay in Knuckles in Sonic 2.
When a non-Sonic game (or Sonic the Hedgehog) is plugged in, the result is a game of Blue Sphere special stages.