I have my running backs do this drill as often as possible. It works on several issues at the same time.
Running Back Drill
The running back gets into his stance (3 point or 2 point). Two other players stand about 7 yards away with hitting bags or just in full gear.
The running back comes out of his stance, runs past a quarterback who hands him the ball. Then he needs to run in between the two players who are about 2 yards apart.
The players need to bring their shoulders inwards so that it is hard for the RB to run through. He of course needs to get low and pump his feet once there is contact. He needs to get through with the ball.
Then the coach points out the imaginary end zone is about 12 yards behind the two players. This means that the RB needs not only get through them but stay on his feet so that he can continue to run to the end zone.
Next, you have the imaginary end zone 3 yards behind the 2 blockers. Now, the RB just needs to get through, even if he just falls into the end zone. It requires different skills than the previous setup.
Once they go through both versions, you once again have the end zone 12 yards away.
The coach stands behind the RB when he is in his stance. When the RB is running towards the blockers the coach signals the blockers to move away from each other, leaving a free pass for the runner.
At this point, the RB should still stay on his feet and not fall, as the end zone is still far away.
Each RB goes 4-8 times without knowing when you signal to the blockers to open the gap. A good RB will stay on his feet even if the blockers suddenly move apart. At the same time, if they stay put he should hit them hard and low, pumping his feet, and getting though.
Good luck and enjoy.