The correct answer to the Release to Flatback quiz is the skill is first Legal in Level 6. In Level 1 Release Moves are not allowed, and in Level 2 only log rolls are allowed. In Levels 3-5, where Release Moves are allowed, the following rule was added:
Release skills that land in a non-upright position must have 3 catchers for a multi-based stunt and 2 catchers for a single based stunt.
This rule makes the Release Move in the video Illegal in Levels 3-5. The rule was not added to Level 6, making Level 6 the first level the skill in the video is allowed as performed.
The explanation is based on the 2017-18 version of the cheer rules dated August 30, 2017.