It does nothing in real football terms to explain why it fixes the left rusher position. What it does do however, is form less of a pocket by having the tackles engage their players quicker. The angle that they left rusher engage changes and the left rusher no longer is able to simply walk around the tackle on many occasions. This actually helps the RE/ROLB's sack numbers by a ton because now when they make a move and win (which happens more often than you'd think) they don't immediately get caught back in the loop like they would with a deeper pocket (assuming you step up).
Now if you step up properly against the 4 man rush without pass protecting aggressive you can also fix that problem but if you are running a 7 step drop or simply aren't comfortable stepping up blind (since you should be looking at WRs) it can be difficult to manage. If you get good at it though you can start slide in the pocket and the cpu DEs will alternate between going inside or outside the tackles based on your position so you can work them to your liking using that method. You might take a few more DT sacks and you can also take the occasional butt sack but I have trained myself to step up on just about every pass play now since it helps fix a lot of issues within the game as well as making it so that your sacks don't kill you.
There are a ton of real concepts and techniques in this game however EA doesn't bother telling anyone about them. You can play a squat man technique 10 yards off the line to take away out routes and deep comebacks however you have to know that there are multiple techniques of off coverage in this game.
If you pick a 2 deep man defense then the DBs play a trail technique in their normal press coverage but if you back them off they will play a squat technique and undercut many routes that beat normal man coverage. If you turn that into a Cover 1 shell you'll get burned deep however because they won't turn their hips in time to run with a 9 route.
If you pick a 1 deep man defense the DBs now play a bail technique when in off coverage. The moment a WR gets 10 yards downfield they turn their hips and bail out in order not to get beat deep. This is why curls and out routes decimate most off man defenses, by the time the receiver cuts back the defender has already bailed to avoid getting beat deep.
The gameplay within the game isn't quite as flawed as everyone seems to think. Yes it has major problems but there is also a huge amount of stuff that is done right even if EA hides a large portion of it from us. I had no idea they had any form of the TE Stunt that actually worked in this game till I started messing around with finesse rushers. It was a big complaint of mine that I couldn't use a TE stunt with Mathews. Its still a problem EA thinks its only a finesse technique but the fact that its there was a pleasant surprise for me.