Explosive pull from a dead start, no floor reps needed.
Stay patient off the blocks, the speed comes from the hip snap not the arms.
Starting from blocks just below the knee deletes the pull from the floor, so the entire rep hinges on a violent hip snap from a dead, hinged position where your hamstrings are loaded and ready, with your glutes firing to finish the extension. There is no momentum to lean on, the bar has to accelerate from zero right at the power position. Coaches use it to sharpen top-end explosiveness without the fatigue of full pulls.
Blocks isolate the explosive top half of the pull and remove the slow first pull entirely, so you get more quality reps at the position where speed is actually made. They also spare your back some fatigue.
Just below the knee for this variation. Higher blocks shorten the pull further and bias pure hip snap, lower blocks reintroduce more of the pull from the floor.
Similar bar position, different demand. In a hang clean you hold the bar and can sneak in a countermovement, while blocks give a true dead start with no stretch reflex, which is exactly what makes them hard.
