The torque produced in the coil due to the current in the coil which opposes the rotational motion of the coil is called the back motor effect.

Explanation

A large turbine is turned by high pressure stream or waterfall. The shape of the turbine is attached to the coil which rotates in the magnetic field. It converts the mechanical energy of the turbine to electrical energy. The generator supplies current to the external circuit. The devices in the circuit that consumes electrical energy are known as the “loop”. the greater the load the larger the current is supplied by the genertor.

When the circuit is open, the generator does not supply electrical energy, and very little force is needed to rotate the coil. As soon as the circuit is closed, a current is drawn through the coil. the magnetic field exerts force on current carrying coil.
In the figure, force F1 is acting on the left side of the coil as an equal but opposite force F2 acts on the right side of the coil.

The ese forces produce counter torque that opposes the rotational motion of the coil. This effect is called the back motor effect. The larger the current dawn, the greater the counter torque. This means more mechanical energy is required to keep the coil rotating with constant angular speed. The power consumed by the “load” comes from the ” energy source” to drive the turbine.