Sonic the Hedgehog was the mascot for Sega during the 16-bit console war of the 90s, with Nintendo and Mario in the other corner. Sonic was meant to be seen as a faster, cooler alternative to the slower pace of the plumber’s adventures. The first Sonic the Hedgehog game was released in 1991 which was then followed by two more sequels in 1992 and 1994 but the game we’re going to focus on, also released in 1994, is Sonic & Knuckles and its lock-on technology.
The character of Knuckles was originally meant to be included in Sonic 3 but due to time constraints he had to be removed and from this the Sonic & Knuckles project was born. What is unique about this game cartridge is that it includes a slot on the top where another game can be inserted. This means that games can “lock-on” together and you will be able to access new features. So whilst the Sonic & Knuckles cartridge featured a game in its own right, plugging in a copy of Sonic 3 would unlock Sonic 3 & Knuckles, inserting Sonic 2 would allow you to play through that game as Knuckles and plugging in any other cart would result in Sonic, Tails, Knuckles, and Robotnik telling you that the cartridge is supposedly incompatible but pressing the A, B and C buttons would unlock an extra mini-game.
The main differences by plugging in a game happen in Sonic 3 & Knuckles so let’s take a look at that one. The main advantage is that you can play through Sonic 3 as Knuckles but other benefits include playing as Tails in Sonic & Knuckles, merging Sonic 3 and Sonic & Knuckles into one continuous game and changing a few gameplay elements. Basically the lock-on technology allowed the developers to create the complete game they wanted, which is why Sonic & Knuckles with Sonic 3 plugged in is often referred to as Sonic 3 Complete Version.
I've had a look around the net but can't find the cart being taken apart to see the printed circuit boards that make the whole lock-on technology possible, so if anyone finds any links please post them below.