This seems like a good place to mention that this decocking method won't work on every crossbow. Some crossbows won't release the string unless an arrow is detected. This helps prevent that scourge of crossbow shooters, dry firing, but it also prevents you from decocking your crossbow in the way I'm describing.
In such cases, you should follow the manufacturer's instructions for decocking your crossbow. Some folks have used dowels to "trick" their anti-dry-firing devices, but of course if you try that you must do so at your own risk.