The below the knee rack pulls is fantastic for overloading the glutes and developing a powerful hip extension. Due to the reduced range of motion when compared to a regular barbell deadlift you can use a greater amount of weight to overload the muscles in a range of motion more specific to a vertical jump. It can also be easier on the lower back for the same reason.
You can use heavier weights for developing pure strength, or go lighter for building explosiveness.
The key things to focus on are:
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