What is the key feature of the hydraulic swivel (rotary manifold)?

The hydraulic swivel, also known as a rotary manifold, is a critical component in crane operation because it enables the boom to rotate continuously without the risk of twisting or damaging the hydraulic lines. This design is essential for the functionality of cranes, as it allows for smooth and unrestricted movement of the boom, facilitating tasks that require continuous rotation. The innovative engineering of the hydraulic swivel maintains the integrity of the hydraulic system while ensuring that the operator can maneuver the crane efficiently, providing both safety and versatility in operation.

While the other options provide useful features found in various hydraulic systems or crane operations, they do not specifically capture the unique role of the hydraulic swivel in allowing for continuous rotation without line entanglement. Thus, option B highlights the most critical function of the hydraulic swivel, making it the correct choice in this context.