Motor unit synchronization is a fancy word that is used to basically describe the timing of your muscles firing. When muscles fire at the right time, we have better “synchronization” and waste less energy. So as we train and perform a movement, we get better and better at doing that movement. Yes, some of it is obviously the fact that we are getting stronger, but some of the gains come from how your nervous system organizes the movement itself.
So when people talk about “neural adaptations” occurring early on in training, what that includes is how efficiently your brain does the movement. This is why practice really does matter, because you teach your body how to use your muscles more efficiently. It also means that when we train a new exercise, we need to appreciate that we aren’t always going to be as strong as we think at first because our body might not yet be used to performing that movement yet.