As far as I'm aware, load banks were used to test diesel-electric locos, not to test OHLE.
Diesel-electrics use a diesel engine to drive an electric generator, which in turn powers the loco's motors. In order to test the 'generator' part of the loco on its own, without moving the loco, a load bank was connected to the generator's output in place of the traction motors. The diesel engine/generator could then be subject to numerous tests and loading scenarios without leaving the workshop. As edwin_m says, a load bank is effectively a giant resistor, which dissipated the generated power.