Lugging an engine

[Padeye]

This thread http://boards.straightdope.com/sdmb/showthread.php?t=279472 made me wonder about another issue, lugging an engine at low RPM. I’ve been told all my life not to do this, particularly with small displacment engines. I know that in some conditions that loading an engine at low RPM might result in detonation but aside from that what is the mechanism for engine damage or excess wear if any?

[astro]

Seems to be primarily a damage via “prolonged & heavy detonation” issue

WHAT IS DETONATION?
Detonation (also called “spark knock”) is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.
Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.