I've explained many times how the Rams exploited the Cowboys scheme.
I made the videos to go with the explanations.
Most of the Cowboys run defense problems are against Zone Blocking.
The DL flows to the play-side on Zone Runs. Jumping up-field has not been a big issue on zone runs.
The issues are about lateral movement and blocking assignments.
Zone runs start to the outside and then often cut back inside.
Defenses have to balance how much they move outside (play-side) vs how much they stay put to defend the cut back.
The Rams literally always (all but a couple of plays) to the side where the 1-tech was aligned.
They did that because the open gap on that side is the B gap (OG - OT).
If the 1-tech has to move across the extra gap as compared to when the run is to the side with the 3-tech.
The Cowboys 1-tech was making a mad-dash to get outside (i.e. He took himself out of the middle area).
That made it easy to block for the inside cut back.
They just needed a good double-team on the backside DT which was the 3-tech.
With the 1-tech taking himself out of the area, it was easy for an OL on that side to kick out and block a LB.
One of the two OL doubling the 3-tech could kick out and block the other LB because the timing of cut back plays means a delayed block works perfectly.
It was a similar concept with the backside LB. If he stays in place looking for the cut back or often looking for some backside misdirection including a QB keep and roll out backside, then the OL kicks out and pins him to that side. If that backside LB moves hard to the play-side, then the OL will box him out on that side which is even better because it boxes out box LBs to that side basically with 1 block.
Patriots vs Rams
Belichick changed his defensive alignment to play the Rams.
He moved a LB outside and up on the line.
That allowed the DE on that side to play the gap inside the OT instead of outside the OT because the LB would play the gap outside the OT.
With the DE playing the otherwise "open" B gap, the need for the 1-tech to move quickly to the outside was limited.
The trade-off was that it put that LB on the outside in coverage where the Nickel CB would normally be located (they flipped the Nickel "slot" CB to the opposite side to balance things out). The LB would play a short zone and a Safety would cover over the top. The CB on that side would cover the outside WR as normal.
Summary:
Zone Blocking teams are gashing the Cowboys by boxing out their players (especially the LBs) laterally (side to side).
Rams: Primarily by showing outside zone run then cutting back inside.
Vikings: Primarily by outside zone runs but then an occasion inside cut back if the Cowboys started to cheat to the outside.
Belichick made a simple change in this defensive alignment moving a LB to the outside.
That LB on the outside was responsible for area. He didn't need to move far laterally.
The remaining LB on the inside didn't need to move hard to the outside because there was already a LB there.
Your (buybuy) concept of letting the defender go where he is going was true, but it's more about letting the DT move laterally which takes him out defending the inside cut back or if he does not move laterally quickly, then and outside gap will be open.
The RB "reads" the backside LB and the play-side DT then either stay to the outside or cuts back inside based on their movement.
The Belichick alignment mucked up the "read" for the Rams RB and without their run game working, it was easy to attack their passing game.
