The method of balancing on and down moving thin beams.
Grinding (グラインド,Guraindo?) 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.
In later games, such as Sonic Heroes, Power Type and Flight Type characters can also grind. In addition, the characters don't need to wear Soap Shoes in order to grind. Also, games before Shadow the Hedgehog required the use of balancing on the rail via control stick in order to properly maintain speed. This was removed possibly due to need for simplicity and possibly the fact that Sonic and his friends grew more experienced (along with the fact as of Shadow the Hedgehog, rails have somewhat expanded in size and are less thin).
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.
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 and City Escape.