A torque strap is essentially a connection between the engine and the frame that prevents or lessens the movement of the engine caused by the torque transmitted by the crankshaft. Have you ever watched the engine rock back and forth in its cradle? That is caused by torque.
Pros: engine mounts last longer as they do not suffer as much, more power is delivered to the transmission.
Cons: My torque strap makes a faint click when the motor pulls against it, more vibration is transmitted to the cab.
I made my torque strap from 3' of chain, a large eye bolt, and a smaller bolt. Cost about $6 from Home Depot.