If we take a 1:1 system and turn it upside down it will result in a 2:1 mechanical advantage. Instead of the pulley being attached to an anchor it is now attached to the load (pulley A).

On one side of pulley A the rope has been attached to a fixed anchor point, the rope on the other side of pulley A has been sent back down to the ground via a redirect pulley (pulley B) where the user applies the effort to lift the load.

As the load is being supported by two sections of rope (via pulley A), each rope will bear half of the loads weight or 50kg in this example. Pulley A is being subject to the full weight of the load (100kg).

The directional or redirect pulley (pulley B) supports half of the weight of the load (50kg) on one side but an effort of 50kg is being applied on the other side to raise the load, so pulley B and its anchor are actually being loaded with 100kg.

Diagram showing a pulley system with a 2:1 mechanical advantage