When you use the auto function in Practice mode, the game adjusts the tempo based on your playing. Once you finish a 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 for the stringed instruments and piano, tap or click the screen and toggle on Auto-adjust speed from the options. Select Done to save the setting.
For singers, to access the feature, tap or click the screen and select Tempo from the options. Select the Auto tempo button to toggle it black.
Watch this video for more information about it.