S is one of the many monitor power-ups. Unlike many other power-ups, monitors with S on it aren't featured normally in-game and are seen in earlier Sonic titles for Sega Genesis. Because of this, the player is able to find them only after activating the Debug Mode. It also noticeable that monitors with S on it don't usually work in several games while they are discovered as unused content of the game. Most of the time, by breaking this monitor will automatically induce a Super transformation.
Monitors with S are first seen in Sonic the Hedgehog 3, where the player has to activate the Debug Mode. When the player inserts empty monitor to the zone, it apparently turns out to be monitor with S on it. With playing as Sonic, after breaking the monitor, the player will get automatically 50 Rings to the hand and the player will transforms into Super Sonic without collecting any Chaos Emeralds from Special Stages. If the player does this while playing as Tails, the transformations vary, even glitching sprites of the playable character, as Tails is not invented to have Super form in the game. In the locked on version of Sonic the Hedgehog 3 & Knuckles, S Monitors are found via Debug mode. By breaking it, it will automatically turn Sonic and Knuckles into Hyper Sonic and Hyper Knuckles, while Tails is able to turn into Super Tails. All of them also get 50 rings as well.
However even in earlier games these monitors are seen to be used, but have become unused and are dropped during the game's development. In original 16-bit version of Sonic the Hedgehog, unused sprites of monitors with S on it are discovered. According the issue of Computer & Video Games, the player would be able to special power-up, which could have both Invincibility and Speed Sneakers at once. This would lead the possibility, that the S power-up would be featured fully functionally at some point during the development. The same power-up is years later used in 2013 re-release of original Sonic the Hedgehog, where it can be found by using the Debug Mode. Similar to Sonic the Hedgehog 3 & Knuckles, it can make the playable character to transform automatically into their Super form with 50 rings.
One of earlier prototypes of Sonic the Hedgehog CD titled "Prototype 510" has also monitors with S on it, which are identical to unused ones from the original Sonic the Hedgehog. These monitors are seen normally lying around playable zones of the game, as the player breaks these, it will increase the speed and gives invincibility, like Computer & Video Games issue apparently stated.
- Due to Sonic and Knuckles sharing different color palettes in Sonic the Hedgehog 3 & Knuckles, the monitor has S colored red instead of blue, when the playable character is Knuckles. The water shield shares this trait.