When using cranes, what is meant by the term "double blocking"?

Double blocking would reduce the effort required to lift an item but would reduce speed at which the lift was achieved.
Also known as double lufting(this is not lifting spelt wrongly)
Double blocking is when the the hook/block assembly is dragged into the head sheave, which can cause all kinds of trouble.

The crane should be equipped with safety features that will prevent this from happening.

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