Ray Allen, being guarded by Jameer Nelson, jogs from the perimeter to set a pick on Shane Battier’s defender, Beno Udrih.

After Allen sets his pick, Nelson and Udrih switch.  It looks like Battier is going to come off of an Allen-Birdman double screen (probably for a three pointer), so Jameer Nelson shades toward his right to cut off LeBron’s passing angle.

However, Battier doesn’t use the double screen and instead slips between the double-screen.  This play is probably a “read-option” type play where if Nelson is cheating the double-screen (like now) Battier slips between the double-screen, but if Nelson is not cheating, then Battier uses the double.  Because Jameer Nelson was cheating the double-screen he is caught a little out of position, which means that Udrih or Birdman’s defender, Tobias Harris, are going to have to help.

Battier runs into Harris, giving the Birdman a little separation from his defender.  Meanwhile, Udrih has stepped into the paint and away from Allen, to help on Battier.  At the same time, Allen has begun his motion to step out and receive a pass from LeBron along the perimeter.

Battier has taken Nelson and Harris away from the action and forced Udrih to help a little too.  Now that Birdman doesn’t have Harris defending him, he can set a screen on Udrih and not worry about having Harris step out and challenge Ray Allen (who is the NBA’s all-time three point leader, and who is also standing all alone on the three point line).

The fact that there are two players clustered around Battier shows the effectiveness of his decoy.  Birdman sets his screen and Udrih tries to recover and fight through the pick…

…but instead, ends up falling on his face.  Meanwhile, Ray Allen rises with his perfect shooting motion…

…and hits the uncontested three.

If you want to see it in real time: