A deficit deadlift is performed by standing on a mat or plate usually 1 or 2 inches thick when performing the deadlift. It is primarily using for improving speed off the floor because you have to pull the weight through a longer range of motion and are pulling from a position which is more difficult than a regular deadlift.
Most people think that the deficit deadlift only helps improve deadlift strength from the floor but can it also help improve your lockout strength when you do it with good form.
I’ve talked before about how losing upper back tightness and a poor start position are some of the things that make the deadlift lockout more difficult.
If you can hold a good upper back position and pull from a deficit all the way to lockout, while holding a good back position, you are now holding a good position through a larger range of motion with theoretically more time under tension due to the longer pull. This will teach you to hold and strengthen a strong braced position which will transfer to your pull from the floor.
You can take it one step further and add a pause. When deadlifting from a deficit, pause where you would normally break the floor to help you improve your starting position in the deadlift.