API里面这么说的：

This method is called before an activity may be killed so that when it comes back some time in the future it can restore its state. For example, if activity B is launched in front of activity A, and at some point activity A is killed to reclaim resources, activity A will have a chance to save the current state of its user interface via this method so that when the user returns to activity A, the state of the user interface can be restored via  onCreate(Bundle)  or  onRestoreInstanceState(Bundle) .

也就是在销毁一个Activity之前调用，估计书中说的“ 启动 ”是先finish再startActivity，跳转直接就是startActivity，这样就能说的通了。