[Scrapper1]

Originally Posted by Snaqwells

A defender can't be in the lane alone for more than three seconds (if he's guarding someone, it's okay).

Snaq has it pretty close, I think. The penalty is a technical foul, btw.

Here's the rule from the NBA's website:

Section VIII-Defensive Three-Second Rule

a. The count starts when the offensive team is in control of the ball in the front-court.
b. Any defensive player, who is positioned in the 16-foot lane or the area extending 4 feet past the lane endline, must be actively guarding an opponent within three seconds. Actively guarding means being within arms length of an offensive player and in a guarding position.
c. Any defensive player may play any offensive player. The defenders may double-team any player.
d. The defensive three-second count is suspended when: (1) there is a field goal attempt, (2) there is a loss of team control, (3) the defender is actively guarding an opponent or (4) the defender completely clears the 16-foot lane.
e. If the defender is guarding the player with the ball, he may be located in the 16-foot lane. This defender is not required to be in an actively guarding/arms dis-tance position. If another defender actively guards the player with the ball, the original defender must actively guard an opponent or exit the 16-foot lane. Once the offensive player passes the ball, the defender must actively guard an opponent or exit the 16-foot lane.
PENALTY: A technical foul shall be assessed. The offensive team retains pos-session at the free throw line extended. The shot clock shall remain the same as when play was interrupted or reset to 14 seconds, whichever is greater.