Grinding

Grinding (グラインド) is a recurring gameplay technique in the Sonic the Hedgehog series. Grinding is the act of balancing on a rail or some rail-like object. By doing this, playable characters can gain more speed/momentum while on the rail or take shortcuts through the levels.

Overview
Grinding first appeared in Sonic Adventure 2 in which the only characters who could perform the technique were Sonic and Shadow. Even then, they wore customized shoes similar to the Soap shoes to allow them to do so. Grinding is an essential element in this game, and is not only useful for cutting time off one's record, but was also necessary for crossing certain points in levels and was also used as a mechanism by which to aid in defeating more difficult foes.

In all subsequent games starting with Sonic Heroes, all playable characters have made use of the technique, and have done so with ease. Moreover, Soap Shoes are no longer needed to facilitate the process. However, conscious effort in balancing on the grinding surface is required in order to maintain speed. By the time of the debut of Shadow the Hedgehog, this was no longer necessary.

Grinding is also present in the Sonic Riders series where the characters ride on Extreme Gears to perform the move. In Sonic Riders, only Speed Type characters (and other Type characters riding on certain Gears) were able to do so. In Sonic Riders: Zero Gravity, characters whose Gear were Skates or Boards with the Grind Gear Part were able to do so. In Sonic Free Riders, the Grind Skill Gear Part alone can be set for almost every Gear (save for certain Gears) including Bikes.

Recurring mechanics of acceleration
In Sonic Adventure 2, if pressing, the character will crouch in order to gain speed. However, in Sonic Heroes, Shadow the Hedgehog, and Sonic 06, if the player presses //, the character will do a 180 degree spin which will accelerate them on the rail. In Sonic Unleashed, Colors and Generations, Sonic can move through rails fast by using the Sonic Boost. In Sonic Unleashed (PS3/Xbox 360 version) and Sonic Generations (PC/console version), the 180 degree spin can be performed while grinding for no effect by pressing /.

Grinding once again appears in all versions of Sonic Generations. However, it not only appears in stages that originally had rails, it appears in some stages without it in the past, such as Mushroom Hill.

Sonic X
Grinding also appears in the Sonic X anime series, where it is only used by Sonic. Sonic's first usage of this move was in his battle against Chaos 6 when he grinds on his water tentacles. The move's next appearance was in "Desperately Seeking Sonic", only this time, Sonic used a pair of shoes created by Chris to grind on long tree branches to reach Eggman.

Trivia

 * According to Sega, the grinding mechanic's addition to the Sonic series was heavily inspired by the fact that they were working on Jet Set Radio, which originally used the mechanic, around the same time Sonic Adventure 2 was in development.
 * Classic Sonic doesn't "grind" in Sonic Generations, more he just rolls along the rail. Bizarrely (and somewhat painfully), this still causes sparks and metallic scraping. This is more noticeable in Crisis City, City Escape and Planet Wisp.
 * In Sonic the Hedgehog (2006), Silver cannot grind in single player, but he can in multiplayer.
 * Amy in Sonic Boom: Rise of Lyric has a similar move called the Beam Walk, though she walks across instead of sliding along them.