Disney's "Envelope Of Protection"

"How Disney Ensures That Your Arms Won't Get Torn Off On Rides":

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'...

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... 

Interestingly, "selfie sticks" can extent beyond the "envelope of protection" and cause serious problems. Hence, Disney has banned them from the park.