The TH Marine Torque Tabs help to reduce or eliminate steering torque caused by the propeller rotation and transom height.
 
It also reduce lower unit "crabbing" which is caused by the propeller twisting the gear sideways causing it to run through the water sideways, slowing the boat down. This positive effect is created by the hydrodynamic pressure generated from the tab.
 
They should be installed by a professional welder but there is a provision to install less expensively for nuts and bolts for boats under 50 mph. 
 
The plate is made of aluminum and comes with instructions and stainless steel installation hardware.