Runner Down

A ball carrier is down when any part of his body, except his hand or foot, touches the ground (Rule 4-1-3b).
The hand/wrist and foot/ankle touching the ground does not put a runner down.
The first reviewable situation is whether a player made the line to gain or not, either for a first down or the goal line. If the runner is ruled down, replay can review that spot in relation to the line to gain. If the line to gain is not involved then that spot is not reviewable.
The second reviewable situation involves a runner not ruled down and allowed to advance by the officials. Replay can always look at whether the runner should have been down by rule prior to the advance. If the officials rule the runner down replay can only look at the position of the ball at that point in relation to the line to gain or the goal line. The one exception is if the runner is ruled down but crosses the opponent's goal line in the immediate continuing action after the ruling.
A ball carrier who obviously has given himself up and is sliding feet-first is down and defenseless (Rule 2-27-4).
As soon as a player starts his slide (typically indicated by the player's butt starting to drop), the play is over and the runner is down.
A fumble cannot occur after a player starts his slide.
It is a fumble if a ball hits the ground and comes loose with no other body part down. A ball that comes out with only a hand or wrist touching the ground is a fumble.
A foul against a player after he began his slide is a dead-ball foul. On fourth down, the ball turns over to Team B and the penalty is enforced after the change of possession.

Runner Down

A ball carrier is down when any part of his body, except his hand or foot, touches the ground (Rule 4-1-3b).

The hand/wrist and foot/ankle touching the ground does not put a runner down.

The first reviewable situation is whether a player made the line to gain or not, either for a first down or the goal line. If the runner is ruled down, replay can review that spot in relation to the line to gain. If the line to gain is not involved then that spot is not reviewable.

A ball carrier who obviously has given himself up and is sliding feet-first is down and defenseless (Rule 2-27-4).

As soon as a player starts his slide (typically indicated by the player's butt starting to drop), the play is over and the runner is down.

A fumble cannot occur after a player starts his slide.

It is a fumble if a ball hits the ground and comes loose with no other body part down. A ball that comes out with only a hand or wrist touching the ground is a fumble.

A foul against a player after he began his slide is a dead-ball foul. On fourth down, the ball turns over to Team B and the penalty is enforced after the change of possession.