The cable lifting apparatus on a crane passes through the pulley block at the tip of the boom, and one on the hook. Like any pulley system this gives a mechanical advantage to the hoist, allowing heavier lifts. As the hoist is located on the stationary superstructure, when the boom telescopes outwards (or is lowered), the hook block will be pulled towards the boom end as the cable effectively shortens. The "two-block condition" occurs when the hook block runs into the boom block, snapping the cable and causing the load to drop. Needless to say, this is a Very Bad Thing. It can be prevented by spooling out cable fast enough to match the extending boom. Modern cranes have mechanical sensors which alert the operator if the two-block condition is imminent.