At the beginning of the game, there is a coin toss between the two teams’ captains. The winner of the coin toss decides if her team will start the game kicking (on defense) or receiving (on offense).
During the kick off, the team that is kicking stays in a straight line behind the kicker’s last foot. As soon as the ball leaves her foot, the team runs up and attempts to tackle whoever caught the ball.
The team that is receiving the ball is in a formation called the exploded scrum. This formation ensures that the receiving team is spread out enough to catch the ball wherever it goes and close enough that when someone gets tackled, she has teammates nearby to ruck.
When a player is tackled, a ruck is formed. The one tackled only has a few seconds to place the ball towards her team before she has to release it. She then wants to roll or crawl away from it as soon as possible to avoid getting stepped on by either team. The players in the ruck try to keep possession of the ball by having it stay between her feet until the scrum half is able to pick it up and pass it to her team. The other team tries to do the same. When rucking against someone, one wants to get lower than the player she is rucking against with knees slightly bent and a straight back, one foot in front of the other. Other players from her team then support her by placing her shoulder on the first rucker’s behind and pushing until the ball on the ground is between her feet.
The ruck is over when the ball is out. A good way to tell is that if a hypothetical bird can poop on the ball because it is not being contained by a player, then the ball is out.