If you're feet aren't supporting your weight, you are "off-your-feet". If you are off your feet you are not allowed to participate in play, and in fact have a duty to roll away from the ball.. Ergo -- knee on the ground means that you are not allowed to play the ball in any fashion whatsoever. It could also refer to a player engaged in a ruck who (although still on their feet) plays the ball with their hands (not allowed) rather than rucking it backwards with their feet.
When your on your knee is on the ground, you are considered on the ground which means you are out of the game and cannot touch the ball. As soon as you're tackled you're allowed to place the ball back to your team but after that you have to release it. If you are the tackler, you have to get onto your feet first and if you're the first person there you're allowed to pick it up. As soon as when you are in the ruck and you have more than just your feet on the ground, you are out of the game.
