Yes, you are misunderstanding.
The idea was that your body can produce X amount of force for, lets say - 10 reps.
The rest interval should be long enough so you can produce SAME amount of force, as producing force agains this object that weighs that one specific amount is what stimulates the best growth.
Under-resting and not being able to produce as much force as you already know you can, leads to upper thresh-hold fibers not firing as much, therefore creating less stimulus to growth.
What stimulates growth is the upper threshhold fibers, but those only fire up when you are under a certain intensity. Thats why drop sets dont work, because you are feeling the burn and you are tired because you are just skipping rest, and only fibers that work at that point are the endurance fibers.
To iluststrate it, lets use 10 reps of 10 rep max.
So we know there are 3 types of muscle fibers. Slow oxidative (SO) , Fast oxidative (FO) and Fast Glycolytic (FG).
The first ones cant contract super fast but they can maintain the work for long time, while the last ones can contract like a mothafucka but only for a rep or two.
Also the first ones dont need much rest while the last ones need super long rest to be able to fire off for that 1-2 reps.
So when you do 1-3 reps of a set at the beginning, the only fibers working are the SO. When you do 4-8 reps, the SO and the FO. And only the 9th and 10th rep of the set is so friggin hard, that the FG fire off, finish the set and send the signal to your body that “this was heavy enough, we need to build more of US, so we can do this work easier next time”.
If you lower the weigh, do drop sets, or dont do 10 reps again with the same weight, you repeat the process i described but you never get to the point where FG gets to fire off as those will only fire if ALL conditions are met :
1)They had enough rest and enough energy
2)You are under the same tension where they are needed.
Of course, when high amounts of IGF, HGH and anabolics come into play, the metabolic fatigue(just jerking off for lots of reps and sets with light weight) also produces stimulus for growth, but its still not the most optimal way. Its just that for some people genetics and drugs do the most work. Its not that they train smarter, its just that they would have the same result no matter what they are doing.
Again, i am not an expert and i havent accomplished shit in this field, i just express my opinion on the topic based on how i understand the current materials on the exercise science topic, so dont bash me, if i said something completely stupid.