How?

When lifting from the blocks, the pressure on the feet prior to the bar being separated from the blocks will need to be farther back toward the heels than it would be during a lift from the floor when the barbell is at the same height, or in the starting position of a hang power clean from the same starting height. – Catalyst Athletics

The block clean will force the lifter to accelerate the bar more rapidly because of the limited distance available to accelerate, and because it’s beginning from a dead stop with no prior stretch or tensioning of the lifting muscles as would occur from a hang position. This means it can be a good choice for training speed and rate of force development. It can also be used during periods of time when the loading on the back and legs needs to be reduced somewhat. Additionally, lifting from the blocks rather than from the floor reduces the loading on the back and legs, meaning that it’s less taxing on the lifter. – Catalyst Athletics