Broken thumb? No problem.
In fact, this formation may serve to the Steelers advantage.
The Pistol formation, run with a high level of success by the University of Nevada and the University of Indiana, is a hybrid run/pass balanced formation. It's not a full shotgun, which oftentimes leads to the misdiagnosis of its real purpose. It's not about the quarterback standing about 3 yards behind the center (instead of the typical 5 out of shotgun), but rather, it's about the running back standing behind the quarterback, not tipping the defense as to the direction of the play.
The Steelers will look to employ this formation in Week 12 against Kansas City because of Roethlisberger's broken right thumb. That will make taking snaps under center quite painful, and risks damaging it further. One weakness of the shotgun, though, is in the running game. With Mendenhall (or Isaac Redman or Mewelde Moore) lined up even with Roethlisberger about five yards off the ball, runs to the side the back is not on is too obvious and easy to track down for NFL defenses.
Out of the Pistol, the back is still lined up behind Roethlisberger, not tipping off their intentions on any play. He also has the time to be able to hand off with his left hand by utilizing a reverse pivot.
When a quarterback hands the ball off, he does it with the opposite hand of the direction of the play (right hand to left side, left hand to right side). If the run play is called to the left side, he can pivot to his right, and still have time make the handoff with his non-injured left hand. Add in Mendenhall giving a jab step to his right to sell the defense on a run play to that side, he still can generate momentum to the left, and run downhill off the edge - that's when he's at his best.
The Steelers signature run play, called "22 Double," is a power run to the right side of the formation. Kemoeatu pulls to his right, and leads Mendenhall into the hole. Pouncey releases after the snap and blocks at the second level. WR Hines Ward is usually closer to the line (oftentimes in motion from out wide) and blocks the corner or safety. Many of Mendenhall's touchdown runs (particularly the long ones) have come out of this formation.
In the meantime, both Kemoeatu and Pouncey can pull to their left, and TE Heath Miller can block down on the end while LT Max Starks blocks down on the guard, freeing up Pouncey and Kemoeatu. Ward blocks down on the safety, and Mendenhall has a crease in which to use his footwork and speed to beat the linebacker.
Even better, the Steelers can utilize that same concept - reverse pivot, jab step, show run to the left - and run a weak side pass out of it.
Think of the Steelers in their 3-WR formation. Mike Wallace is on the offensive left, on the line. Antonio Brown is wide right, off the line, and Ward is slot left, off the line. Miller is on the strong side (left). Out of that formation, the Steelers can either run or pass and block the same for either one. That presents an issue for the Chiefs, who cannot be suckered by the play fake, but still have to respect Mendenhall and certainly, a pulling Chris Kemoeatu.
With Kemoeatu and Pouncey pulling, but not advancing too far down the field, Wallace and/or Brown sell run, and accelerate vertically. Roethlisberger sells the play fake, pulls back and sets his feet to make a throw down the field.
They can - and will - run both of those plays, probably more than once each, probably to both sides of the field. It's a nightmare to plan for due to the personnel of the Steelers, and an inexperienced and otherwise weak Chiefs secondary will struggle to contain it.
It's also not the first time they've run this. Roethlisberger was in this set often in a Week 14 win vs. Baltimore last season. It didn't have an overwhelming amount of success, but with all due respect to the Chiefs, the Ravens defense, they ain't.
Experience in running it, and defending it, will be key components to this game.