I've got one on my wife's Legacy, the oil used to shoot out of that thing like it was pressurized! It does make for nice clean oil changes (make sure you get the one with the nipple so you can slip a hose over it and drop the oil straight into an old oil container). It won't come open by accident, once you have it in your hand you'll see that it's designed in a way that this is not an issue.
The lever is spring loaded so that you need to push it up and out of the groove, and then turn it to open the valve.