Sonic in an Automatic trail section in Metal City.
The Automatic Trail is a gameplay mechanic used to describe a section of a race course in Sonic Riders that the player riding through it is automatically controlled and requires no input from the player.
In most cases though, the player is prompted to quickly move the control stick in a clockwise direction (or in the case of the PC version, alternately press the left and right arrow keys). If the player performs the prompt, he/she will move faster through the Automatic Trail section and gain Air in the process.