You know that you’re supposed to lift up your arms and scream during a plunge down a roller coaster, but you probably also know you aren’t reaching out as far as you could. After all, it’s hard to ignore the feeling that you might lose a hand if you do so. But Disney, and other theme parks around the world, makes sure that won’t happen with a fairly simple engineering check.

A Redditor claiming to be a Disney employee shared the video below with Jalopnik, demonstrating a ride’s “envelope of protection.” Basically, engineers simulate the absolute furthest a person admitted on the ride could reach out in any direction (and probably much further, for safety’s sake) by sending a wide piece of metal or wood down the track:

As long as the average person can’t extend anything past the envelop, she should be safe. Jalopnik also pointed out that not all envelopes look alike, but follow the same general principle:

The employee claimed that this envelope of protection principle is a big reason why Disney banned selfie sticks from all its parks. Would you really expect not one person among tens of thousands per day to sneak a selfie stick on a ride and impale a fellow passenger?

Selfie sticks: once again foiled by common sense.

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HT: Jalopnik