Here's Kobe Bryant hitting a trick shot from behind the backboard against Oklahoma City:
Although it was counted as good and is being featured on the NBA's YouTube site, the shot was illegal and should not have counted. [UPDATE: evidently, they did change that rule...but someone didn't tell NBA Vice President of Referee Operations Joe Borgia.] Similar shots in the past have not counted. The most famous was by Celtic Larry Bird:
Same shot, different results. Formerly, according to the rules, if the ball passed over the backboard--from either direction--it's considered out of bounds. (There's another famous shot that I can't find on YouTube, where a player was flying out of bounds and made a much longer version of the same shot and it didn't count. Don't recall the player either so Google is no help...)
You could shoot the ball from behind and to the side of the backboard. In that case—where the shooter is not directly behind the backboard—the shot was always legal. The idea is that the backboard projects backwards and extrudes, forming a rectangular block (stick with me on this one) and if you avoid the ball touching that block, the basket is good. You can shoot over the block if you're to the side of it, not if you're behind it. In other words, the ball shouldn't ever be visible through the glass (when looking at the backboard straight on) before it comes over the top.
This is still the position that Borgia related two weeks ago concerning a good shot by Rajon Rondo—that a shot from directly behind the backboard would be illegal. "It’s extremely rare on shots," Borgia said. "Players know it’s illegal."
Kobe: "I was aware that the rule was changed and that you could do that."
Huh? In any case, here's a look at at a good basket:
Though technically it passed "behind" the backboard, it does so from the side, not directly behind. It's somewhat of a technicality, but it's what the officials had formerly (still?) contended is the proper distinction.
Apparently they changed their minds after the Bird shot, but there hasn't been such a clear case as Bird's since then and there still appears to be some controversy. Now that Kobe's made the shot famous again, the cat's out of the bag; they can't go back to disallowing Bird-like shots. Look for this move to be integrated into every NBA playbook. After all, when done right it's impossible to defend!
But maybe the lesson here is that we can't be certain what's going to be legal in the future. Rules can change. Therefore, you should always bring the ball out in front of the backboard like Dr. J:
Just to be safe.