You spot a critical weakness in a foe's defenses, which an ally can exploit to deadly effect.

Encounter
Immediate Interrupt      Close burst 10

Trigger: An ally within 10 squares of you misses with an attack against AC

Target: The triggering ally in the burst

Effect: The target's attack is made against Reflex instead of AC.

Published in Player's Handbook 3, page(s) 173.