Spring is based on a physical model. Each letter is tied to a spring at its center. At the start of the animation all letters are moved to a random positions. The positions can be influenced by minimum and maximum constraints. After the letters took positions, will be pushed by random force. This means a random starting speed. The starting speed can be influenced by minimum and maximum constraints. A special case of this CAPTCHA when the vertical start position and speed is zero.
The weapon of this CAPTCHA: The method is to obfuscate the enemy. If the springs are at maximum distance, the letters can be swapped.