Pulling your shoulders back is messing up your deadlift. Use hinging straight-arm pulldowns to engage your back the right way, optimize tension, and create a more stable hip hinge. And do them BEFORE deadlifting.
It's not uncommon to hear "shoulders back" as a cue when deadlifting. Unfortunately, promoting a shoulders-back position only makes things harder.
You probably follow this cue because it helps you "get tighter" and it stops that hunchback position you sometimes revert to. But by pulling your shoulders back during the setup, you've just made it harder to reach the bar.
By setting your shoulders back and pulling your chest closer to the bar you'll have to get lower to start. Essentially, you've just added an inch or two deficit onto your deadlift.
You've also just activated muscles on your back that are much weaker in their shortened position. Having a ton of load going through your upper back muscles in that weakened position might cause them to break sooner. You may have replicated what many call "safe lifting technique," but you've actually put yourself in a more compromised position.
Instead of "shoulders back," think about pulling your shoulders DOWN. A better cue for this would be to "pull your shoulder blades into your back pockets." This position should be reinforced through accessory lifts that will increase awareness and strengthen the movement.
Straight-arm pulldowns are a highly useful tool for this. They reinforce optimal shoulder positioning and back engagement when deadlifting. Tension and activation in your lats also plays a key role in stabilizing your hip hinge, largely due to its force-producing capabilities and attachment on your pelvis, spine, and thoracolumbar fascia. Strong and engaged lats will stop your back from caving.
To deadlift better, use straight-arm pulldowns in one of three ways:
1 To Prime and Activate
Be unconventional and program straight-arm pulldowns before your deadlifts. Doing these first will increase awareness of your position and the same tension you're creating in your lats when deadlifting.
Don't mistake them for a warm-up though. Train them hard. If you think you'll be too fatigued to deadlift afterwards, then you've got bigger problems.
2 As an Accessory Lift
Use them in either the same session as your deadlifts or on an entirely different day. Extra isolated volume for your lats will bring up the weaker area while reinforcing proper back tension when deadlifting.
3 Pair with Your Deadlifts
If workout efficiency is what you're after, try pairing straight-arm pulldowns with your deadlifts.
If you don't want to leave your deadlifting area between sets to head to the cables, you can even do straight-arm pulldowns using just a resistance band hooked up somewhere close. Go close to failure or just treat it like active recovery.
Broadly, 3 sets of 10-15 reps should do the trick.