When you use the auto function in Practice mode, the game adjusts the tempo based on your playing. Once you finish playing the song, or section you selected, it restarts with the adjusted tempo. The function works from 25% all the way through to 125%, in 5% increments.
To access the feature, tap or click anywhere on the screen and select Tempo from the options. Select Auto so that it turns white to toggle on the feature.
Watch this video for more information about the tempo feature.