Engine Lugging/Engine Damage
Lugging is a relative term, obviously. It's different for different engine configurations. Generally it's running an engine at low rpms while under a load. To go a given speed, a certain amount of energy-per-time (work) needs to put through the motor through the transmission to the ground. When 'lugging", you're causing that amount of work to be done on few cycles (rotations) of the piston/crankshaft assembly. Think of riding a bicycle. It's easier to pull shorter gears (higher rpms) than taller ones. With tall gears, you have to really push on the pedals to go slow. Now think of your piston and crank assembly doing the same thing while you're lugging your motor. All that pressure on the piston crown, piston pin and big end on the connecting rod is just like your quads trying to push the pedal down. The oil film can only handle so much pressure before collapsing and causing metal-to-metal contact. Plus, at lower rpms the oil film will have more time to collapse than at higher rpms.
"I don't know what you do, but I know what I do, and I don't do that." --Uncle Doug, R.I.P.
"Without the possibility of death, adventure is not possible"--Reinhold Messner