what is a three second defensive penalty in pro basketball??

It is when a defensive player stays in the paint not guarding a person for more than 3 seconds. It results in a technical foul and a free throw awarded to the other team. Here's the actual rule from the NBA rules website.

Section VII—Defensive Three-

Second Rule

a. The count starts when the offensive

team is in control of the ball in the

frontcourt.

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) a player is in the

act of shooting, (2) there is a loss of

team control, (3) the defender is actively

guarding an opponent, (4) the defender

completely clears the 16-foot lane or

(5) it is imminent the defender will

become legal.

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 distance 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

possession on the sideline at the free

throw line extended nearest the point

of interruption.The shot clock shall

remain the same as when play was

interrupted or reset to 14 seconds,

whichever is greater.

If a violation is whistled during a

successful field goal attempt, the

violation shall be ignored and play shall

resume as after any successful basket.

when a defender tries to stay in the lane to prevent a play instead of guarding his man