1. Force multiplication: Get a clothes line and stretch it out, holding one end in your hand and one tied to a tree. Have someone put a blanket on the middle of it. You will be surprised at the force created, maybe even being thrown off balance.

2. The ease of altering the path of a moving projectile: It may take a tree trunk (or thick branch) to stop a bullet, but even a twig can redirect it. This is how judo works, and how a few pounds of pressure could indeed curve a whipping cable into a more downwards trajectory.

3. Inertia: The draped blanket will not be able to be accelerated laterally very much by the cable (I’m sure we’re all familiar with the “yanking a table cloth” magic trick?), thus it will apply its force downward much more effectively than, say, a pair of vice grips on the cable. A solid, attached load would just accelerate with it in the direction it wants to go.

Combine all three of these and it makes sense. Common sense (“How could a blanket affect that amount of power?”) is often proved wrong by physics, which is how the magic industry came about. : )