A diesel engine has a torque curve. There is a spot about 75% of full power (throttle set at top rpm, engine lugged down to 75% of top rpm) where most diesel engines develop their top torque.
You can pretty much play in this range without problems.
You can also lug the tractor down lower than this for short periods without problem.
However, if you lug the engine down too low for too long a time, the exaust temp rises. A lot. This doesn't always show up on your temp gauge. Big farm tractors have a seperate exaust gauge. (They burn out a lot - tough environment for them!) Turbos make this worse - or I should say, an even finer line to watch.
Running for a long time in this lugged down condition will overheat the exaust valves, pistons, and so on, and drasticly shorten the life of your engine.
I have no problem at all lugging down my big or little tractors going up a hill. That's short term. No big deal.
If you are going out brush hogging for 3 hours with a hog too big for your tractor, & lug it the whole 3 hours - you are doing damage to it. Note that you do have some room to play with - that power curve where torque rises as rpms slow a little. No problem with that, but if your engine does not respond to more throttle, that means you are lugging it down too low.
The problem with running at 1/2 throttle is that you totally mess up that sweet spot, where the engine gets the power/ torque bulge. It becomes much flatter & smaller, and you have very few rpms to 'play' with any more. It is just so much easier for you to overly lug the engine in this state...
A good general test to see if you are lugging the engine too much is to increase the throttle a little. If the engine has a hard time responding to that, then you are probably overdoing it. Again, for a couple minutes out of 3 hours running time - no big deal. For the whole 3 hours - oh boy! You are cooking it.
Combine manuals from Deere talk a lot about the power bulge and how you can use it for unloading on the go, but you can't use it for constant operation; and farmers who added turbos on to non-turbo engines became real familiar with this; and diesel pickup manuals will talk about it for long hill towing & the like.
How this all applies exactly to a tiny compact diesel - well, I'm just a simple dirt farmer, but I would assume the same basic principles would be there.
--->Paul