The deficit deadlift is a great accessory exercise for either a deadlift or sumo deadlift when performed correctly and programmed for the right reasons.
Let's discuss a few potential benefits
Increased Posterior Chain Work
By increasing the relative distance between yourself and the bar you increase the range of motion (ROM) at multiple joints in order to perform the lift.
This increase in ROM results in an increase in the time taken to complete a rep: therefore increasing total time under tension (TUT). Increasing the TUT throughout the lift increases the workload, particularly in the posterior chain.
Increasing this relative workload for the posterior chain is a great overload stimulus that may result in further strength or hypertrophy adaptations.
Increased motor unit recruitment
Without going into a science lesson, muscle contractions occur due to the recruitment of motor units (a nerve and multiple muscle fibres). Generally speaking, the greater the number of motor units we can recruit, the greater force we can exert during contraction.
By making the exercise more difficult in the first phase of the pull the nervous system will be forced to become more adept at recruiting motor units at these angles; either by recruiting more motor units or increasing the synchronization of the stimulus.
Either adaptation may result in greater force production throughout the lift and assist in starting strength and powering through sticking points.
Increase speed off of the floor
Being slow or weak off the floor is a common problem seen with the deadlift. Increasing the motor unit recruitment as discussed above will definitely assist in improving speed off the floor.
However, deficit deadlifts also force the lifter to use more leg drive! The quadriceps play a huge, under appreciated role in the first phase of the deadlift.
By increasing knee flexion (due to increased ROM) we increase the recruitment of the quadriceps to extend the knee joint. We also increase the queue to drive through the legs, pushing through the floor, rather than trying to solely hip hinge.
The Caveat
Whilst the deficit deadlift can be a fantastic exercise, it needs to be done correctly.
If you cannot get into position with a neutral spine, you likely have some mobility issues you must first address in order to safely perform this.
How should you do it?
Thia should be done with an elevation of 1-3 inches, as long as you can maintain a neutral spine and proper form
The deficit deadlift is perhaps best performed in the 4-8 rep range
Deficit deadlifts are tough on the central nervous system, so it's best performed for 1-2 mesocycle at a time (3-8 weeks).