Try to imagine pushing your feet through the floor rather than pulling the bar up.
Pushing Vs Pulling The Deadlift
The "Pushing your feet through the floor" is incorrect dogma that continue to be perpetuated.
With that said, Pushing or Pulling is determined more so by which Deadlift Technique you use.
Conventional Powerlifting Deadlifters
Dr Tom McLaughlin's (Phd Exercise Bomechanics, former Powerlifter) finding (back in the 180s) demonstrated that in the Conventional Deadlift, the weight is broken off the floor first by the lower back, with the assistance of the legs. Additional research and anecdotal data supports McLaughlin findings
The muscle firing sequence is: "Back > Legs > Back". The back initiating the drive, follow by the legs, with the back finishing the "Pull".
A common term used in referencing the Powerlifting Deadlift is, "How much did you pull?"
That because the Conventional Powerlift Deadlift is a Pull. No one ask, "How much did you Leg Press off the floor?" (drive off the floor with your legs).
With that said, Olympic Lifters who employ the Deadlift for developing Strength for the First Pull, need to drive their feet into the ground, push the bar away from the floor with their legs with a flat back.
The objective of the Olympic Lifting Deadlift Method is to position the bar for the Second Pull, where the greatest Power Output in sport have been measured, which is needed to "Rack the Weight" in both the Clean and Snatch.
Sumo Deadlift
McLaughlin determined the leg drive the weight off the floor.
The muscle firing sequence is: "Legs > Back". The leg break the weight off the floor, the back finishes the final pull.
Take Home Message
Your training objective need to determine the Deadlift Technique that you use.
In other word, "Use the right took for the right job."
Kenny Croxdale