The image below shows a Faraday Disk DC generator. Let's assume that the bearings on the spinning metal disk are frictionless and that we give it a good spin and let go of the handle. I know from the conservation of energy that if the device is allowed to discharge through a load that it will eventually come to a stop. However, it is not obvious to me how a torque is generated in the disk through the action of the electrons dumping their energy out through the load (i.e. light bulb). Thanks in advance for clarification on this.
The induced emf produces an induced current in such a direction as to oppose the motion producing it -Lenz's law.
In the diagram the direction of the conventional induced current is from the central spindle to the rim.
This is the direction of force $F$ in your diagram.
The disc carrying an induced current in that direction feels an upward force which is in the direction opposite to the direction of motion of the disc.
There is the source of the torque in opposition to the motion of the disc.
When electrical power is being taken from the generator, a current must be flowing. That current makes the generator act like a motor, but with the motor torque opposing the rotation. That torque times the rotation speed is power, which in a perfect generator is the same electrical power being delivered.
When the circuit is disconnected, no current can flow, and no backwards torque is developed in the generator. Of course since power is current times voltage, no electrical power is being delivered by the generator either.