You may be told to keep your arms and legs in the vehicle at all times when you get on a roller coaster or ride, but it's unlikely that you'd actually lose a limb if you disobeyed thanks to something called the 'envelope of protection.'

Recently, Disney decided to ban selfie sticks from all their theme parks after California Screamin' had to be stopped due to a passenger pulling out a selfie stick mid-ride. A reddit user who goes by the name 'EnglishMobster' posted in a thread about the ban, claiming that he or she was a Disney employee, Jalopnik notes.

EnglishMobster said that Disney uses an "envelope of protection" when testing their rides to ensure that no one could possibly reach out and hit anything. This sounds like a spell you could cast in Dungeons & Dragons, but it's actually pretty simple. They test the ride using a special vehicle with a disk attached that has a number of spindles fixed all around it. These spindles act as a person's hypothetical limbs, and they're much longer than a human's reach could possibly be.

"This means that when a rollercoaster is going 45 MPH, you can have your hands out and you won’t get hit (it’s not a good idea to test that, however — we do tell you to keep your hands and arms inside the vehicle at all times)," EnglishMobster wrote.

Here's a video of the 'envelope of protection' in action:

Selfie sticks are different, however. EnglishMobster wrote that a selfie stick is an extra three feet, and it can pop out in any direction.

"When you're going 45 MPH, that can seriously damage the attraction and your phone/camera. The selfie stick will likely fly out of your hands and in a worst-case scenario can either hit someone else on the ride (injuring them) or land on the track and derail the sled behind you. One of our largest rollercoasters derailed because someone's backpack fell out once, and we don't want to risk a selfie stick causing the same issues."

EnglishMobster said Disney tried to just ban the selfie sticks from certain rides, but clearly people just weren't listening, so now they're banned from the park. GoPros, however, can be still be mounted to a hand or hat.