To answer the question of 'what' happens:
The connecting rods are not connected directly to the wheels. Because the frames are outside the wheels, the axles extend to cranks outside the frames, onto which the connecting rods are connected. The cranks act like levers and, with the large forces involved, try to twist the axles. When all goes well, this twist translates into rotational movement and the loco moves. Sometimes the forces overcome the cranks and they slip on the axles out of alignment. If not caught quickly enough it leads to bent coupling rods and untold other damage.
A rolling stock expert will be along shortly to correct anything I may have got wrong in what is intended to be a simplified description!