Once selected you’ll be presented with a slider to adjust the velocity. To adjust the velocity (velocity is how hard or soft an instrument is played, plucked hit, etc) of a note first turn off the pencil lock at the top right of the screen then tap the note which will display the velocity option.
You can also utilise Piano Roll with drums: Piano Roll can be used with drums Some people may find using the Piano Roll easier than recording melodies using the virtual instruments, as a smaller screen size on handheld devices may be a hindrance in regards to playing the notes correctly. The Piano Roll displays a grid that allows you to tap in notes corresponding to the keys displayed on the left of the screen. The same process can be applied to GarageBand on the iPhone. This video by Neil Spencer Bruce gives a demonstration of how to access and use the Piano Roll in GarageBand on an iPad.
