manual 上说的很明白的:
void QAbstractButton::clicked ( bool checked = false ) [signal]
This signal is emitted when the button is activated (i.e. pressed down then released while the mouse cursor is inside the button), when the shortcut key is typed, or when click() or animateClick() is called. Notably, this signal is not emitted if you call setDown(), setChecked() or toggle().
void QAbstractButton::pressed () [signal]
This signal is emitted when the button is pressed down.
void QAbstractButton::released () [signal]
This signal is emitted when the button is released.