Common scenario where a block is installed up the tree and you are lowering sections out.
The force at the block is double whatever the weight of the section is ... that's not taking into consideration friction or any dynamic loading if the section was falling.
Imagine you cut off a 100lb section. To stop it hitting the roof you need to apply force to the other end of the lowering rope ... if you apply 100lb then the section is stopped. So you have a 100lb on one side to stop it coming down and you have the weight of the section which is another 100lb. Total is 200lb on the pulley block.
Here's a quick sketch to illustrate.
