This is simply for holding down LT when you’re in open space as a running back or receiver. I did this by using half back toss over and over again until I broke a decent run. As soon as I had space I’d hold LT. This is cumulative, so I would generally get 10-15 yards at a time before being tackled.
I got this early in face of the franchise championship game. Pretty sure it’s on rookie so it’s simple.
*note* - Some in the comments have said this only unlocked for them when they didn’t sim defense. That wasn’t the case for me, but worth trying if it isn’t unlocking for you.