Mouse can be used in 3 modes:
Single-mode: One key can play at a time. Press and hold your left mouse button to effect the duration of the sound.
Chord-mode: Press and hold the keyboard 'Ctrl/Cmd' button while selecting multiple keys with your mouse. Press 'Esc' to cancel selection.
Drag-mode: Press and hold the keyboard 'Alt' button while moving your mouse over the keyboard.