These are known as kneel downs or quarterback kneels. The quarterback kneel is similar to what happens in a chess game when one player realizes that no matter what they do, their opponent will be able to checkmate them in a few moves. It's a surrender, but in this situation, the winning side takes the initiative rather than the losing side. This falls under the category of declaring himself down. This is handled in Rule 7, Section 2, Article 1-d-1: Unlike in the NFL, the ball carrier cannot fall to the ground and then get back up to resume play. When you contact the ground with anything other than your hands and feet, the ball is dead.