Think of it in terms of arousal (not the sexual kind). If you’re training has a lot of max effort, grind it out sets then you’re probably close to your maximum arousal level. What I mean by that is that it’s all hands on deck. The motor neurons are firing as fast as possible and every one of them is firing (max effort and max muscle recruitment).
To contract a muscle a motor neuron fires and innervates all of its muscle fibers. For a simple example, consider doing a leg extension with 50 lbs. It’s easy so you won’t use all of your muscle fibers. You might use 40% of your muscle fibers. This won’t cause all of your motor neurons to fire. To maintain the contraction through the movement the neurons continue firing. They have a rate of firing that can be improved through training. According to my understanding, the same neurons that started the movement continue firing until you stop. It’s like they fire and release and repeat but it’s so fast that you don’t realize it (obviously).
If you have long periods of training where you’re doing this then your CNS starts to fade, or under recover. Symptoms are like overtraining, I guess. You’re tired; feel like crap. You lose motivation. You feel weaker.
You can achieve CNS fatigue through training to failure — it can be max effort singles and doubles or just constantly taking sets to failure.