It's defense 101, but it takes just one player in the box of seven or eight not doing their job or even trying to do a little too much that results in a run seam. It only takes a few looks at one play to show how things should look when each player takes care of their responsibility and what actually happens when one player doesn't.
At the snap the Jags are in a basic 4-3 look with the DL set with 6 tech (Tyson Alualu), 3 tech (Sen'Derrick Marks), 1 tech (Roy Miller) and 5 tech (Jason Babin). At the snap the Rams run a basic lead-ISO play to Russell Allen.
He has B gap responsibility. The G and C play side will double Miller (who has A gap) and chip off to Paul Posluszny.
This is a tough play on Poz because he has to either come over the top of double or fall backside according to backfield flow, but with ISO-read the Rams have created another gap so Poz has to come over the top.
Babin has his play side C gap taken care of and Marks squeezes down the back side B gap while Alualu can manhandle the TE for B gap/C gap responsibility. What you don't see in this photo is the TE is a nub, meaning there is no WR outside of him so the corner is contain as well. So that leaves Hayes free to scrape with the flow of the play and play very similarly to Poz and come over or under the chip of backside G and T.
What goes wrong?
Only one thing.
Geno Hayes does not flow play side over the top of the G/T chip. He actually senses that the play might bounce all the way back outside the TE to D gap and guesses wrong leaving the play side A gap open for a 7-yard gain.
They say its a game of inches because it is. Its easy for us to look back on a game and pick plays out and find the one player not doing his job, but I look at it differently.
I see how close the Jags are to being good.
That's if all 11 do their job.