This is a noise and vibration joke that works by making people think the envelope contains real hatched rattlesnakes. The envelope looks genuine, like a real envelope containing 12 rattlesnake eggs. You wind an elastic band around and around a washer and then the trap is set as the washer lies flat in the envelope. When someone opens the envelope to look inside the washer can unwind and a loud rattle sound is produced as well as vibration. It's quite scary and shocks the life out of you when you don't know its a joke and think there may be baby rattlesnakes which are venemous in the envelope!
The person who sets it off won't know what has happened until you tell them that they've been had!