[darcane]

Technically speaking, lugging an engine is defined as operating it at WOT and decelerating (due to the load exceeding the available torque). This is most commonly done at low engine speeds in high gears, but you could lug an engine at high RPMs by towing a heavy load up a steep hill.

Lugging will cause very high loads on the crankshaft bearings, and very quickly build up heat in your engine. If the engine has suitable bearings and adequate lubrication, lugging, by itself, won't harm the engine.

Often associated with lugging is detonation and piston slapping, especially at low RPMs. Because of the high stress and heat when an engine is lugging, it is more prone to these conditions which are very damaging.