When you have mastered striking the notes by using your wrists you will be able to start rolling. Rolling is achieved by striking a note with your left and right hand alternatively in quick succession. This is done to create the desired effect of a sustained pitch.
This is a new skill so you will need to start off slowly and build up your speed as your wrists become supple.
When rolling, the level to which you strike the notes with each hand should be consistent in order to obtain an even sound. As you become more advanced you will learn how to articulate with dynamics and when to roll at different speeds and patterns.
When you strike the notes the result might be that the expected pitches are achieved, but undesirable tones might be included.
An example of this is when rolling on a note. The result should be a smooth continuous tone, but often the sound is ‘choppy’ or ‘staggered’. Consider continuously plucking and then bowing a stringed instrument: one will sound ‘choppy’ while the other would be ‘smooth’.
Each note has a certain resonance frequency, which is associated with a particular pattern of vibration. Once a note is struck, it will vibrate, producing a sound until the vibration stops, but when you roll a note the sound is sustained. Although the vibration dissipates after each strike, rolling compensates for the loss of sound.