Its easy. What people who are trained poorly are taught like this:
"Ok, now press the gas while at the same time letting up the clutch".
This is a poor way to teach it, what they should say is:
"Ok press the gas first, then after you feel the engine revving up slowly let the clutch out".
One of my ex girlfriends had trouble getting a manual going (stalling) until I told her you don't have to perfectly time both pedals.
Gas first, then clutch.
After a couple of starts it was like riding a bike for her. Now of course when on steep hills you do have to be a little quicker, but the point remains.
You simply do not have to utilize both pedals with the same amount of pressure at the exact same time.
The easiest way to teach someone how to start going in a manual is to purposefully have them rev up to 1500-2000 rpms THEN work the clutch.
You are not going to damage the engine with a low end pre-rev doing this.
Now if you mash the gas in neutral and let it stick at the redline, sure yea, that might fuck your engine up. Generally speaking the lowest redline I've seen is like 6500rpms, so pre-revving to 1500-2000rpm is nothing.
Once it *clicks* before long you can basically match the gas and clutch perfectly without the extra revving, I just don't think peope should be taught to do this from the beginning.