Why is it recommended to use a block when lowering a heavy limb?

Using a block when lowering a heavy limb is recommended primarily to carry the load and reduce the risk of injury to a climber's line at a crotch. When a heavy limb is being lowered, it exerts a significant amount of weight and force, which can put undue stress on the climbing line and the connection points at the crotches of the tree. By using a block, the load is redirected, allowing the climber to safely control the descent without putting excessive strain on the anchor points. This technique not only facilitates a more manageable lowering process but also enhances safety, helping to prevent any potential accidents or damage to equipment that might occur if the load were directly handled without such a mechanical advantage.

The other choices do not accurately capture the primary purpose of using a block. While increasing the height of the cut, stabilizing the climber during the cut, and creating more friction in the line may have some relevance in specific contexts, they do not address the core function of load management and safety during limb lowering. The use of a block is crucial for effective control and risk reduction during this portion of tree work.