As the ball goes to the trigger man, Zach Randolph sets a downscreen for Mike Conley.  This is designed to get him the ball on the top of the key so they can execute their play.

Conley starts to dribble out the clock, waiting for the right moment to pounce.  You can even see Zach Randolph with his hand in the air kind of signaling Conley to “hold on.”

Once the clock hits 10 seconds, the Grizzlies are supposed to start their play, and this is where they screw up.  Sam Young, who was inserted into the game specifically for this play, runs to the low block.  The problem?  It seems like he was supposed to set a cross screen for Zach Randolph (look at him as he tries to call Young over).  My guess was that Randolph was going to curl off the screen and try to get to the rim.  If they doubled him off of the catch, he would have O.J. Mayo in the corner.  However, none of that happens.

Much credit to Randolph here who tries to salvage the play by running over to Mayo and setting a downscreen.

However, there was absolutely no angle for the screen, and it allows for Jason Kidd to get over it easily and deny the pass.

By the time that Mayo gets open to make the catch, there is 2.5 seconds left.

And there is only enough time for Mayo to shoot a running three.  Here it is in real time:

This play was so important because you felt the Mavs taking hold of all the momentum.  If the game was to go into overtime, the Mavs would get the win, and that is exactly what happens here.