Smoke tells the defense a safety is blitzing and Double Smoke means 2 safeties are in the pressure. Here the SS and WS are both blitzing off the edge. Both blitzers will give a "Fire" call to tell the DE to go inside. On all smoke calls the blitzing safety will make a "Fire" call because where there is smoke there is fire. Against a 2 back formation the defense is aligned in a 4-4 look which is identical to their alignment if they were in their Cover 2 (Robber) zone coverage concept.
The offense cannot readily tell if the defense is running double smoke or Robber coverage which creates a natural disguise element for the Horned Frogs without any extra effort. The natural disguise isn't limited to 2 back formations, it extends into TCU's alignments vs. spread formations as well.
With a split #2 receiver to the weak side the WS will make a "Switch" call alerting the ILB to blitz instead.When playing zone against the same formation TCU can play their Blue (Quarters) concept with very similar alignments.
Lastly, against trips the pressure adjusts again to compliment a zone trips adjustment (Special).
And the zone compliment (Special).4-2-5 resource guide. A helpful resource when looking at TCU's defense is the Boise State playbook from 2000. Brent Guy (Boise State Defensive Coordinator in 2000) was on the defensive staff with Gary Patterson at Utah State which is why you see basically identical terminology. Also for more info on the "Special" trips adjustment and the split field safety concept check out Run COD Hit Blog and his articles on TCU's coverage package.