As with any kind of work that involves working with individuals of differing skill levels, ages, challenges, innate abilities, and support systems(to mention only a few), there is no one umbrella answer or prescription to cover all possibilities.
In the case of this high school student who made a pretty significant pencil grip transformation the month previously, there was a one month gap between his last session and the day when I took this video. Over the course of the month, he had AP exams, school field trips, absences, and final exams which made it so that I could not see him for his regular weekly sessions in his school.
So approximately one month after I had worked with him, I saw him and we did some brain gym and I had him write again, and his handwriting grip had actually improved without any additional support.
The basic foundational skills in his hands were improved, therefore when he was using his hands for his daily life tasks such as typing on his computer, dressing himself, eating, playing basketball, the new patterns were getting reinforced and strengthened.
As you can see in the video below, his hands have increased in ease and his non dominant hand has relaxed and become more supportive without working harder.
It does not always happen like this in every case, however these kinds of results happen more often than not. When you break down the activity you're trying to master whether it be pencil grip or learning a new language, down into its component parts and assess and work on the skills that that activity entails, dramatic changes can happen in a short time.