How to teach difficult subjects such as mathematics or physics in a very smooth way ?

I always wondered, why are there teachers who can explain the most difficult subjects such as mathematics or physics in a very smooth way ,so  students understand quickly and get the highest marks in the final exam, knowing that all teachers teach the same curriculum  from the same books.

High school students accept to go to reinforcement centers or buy  explanation summaries for the purpose of benefiting from other teachers who are brilliant in explaining and simplifying subjects, unlike regular teachers from inside the school.

If we do a quick review of one of these explanation summaries or of one of the private classes inside the strengthening centers, we would find that the explanation of the material does not exceed what was explained in the prescribed curriculum book, so what is the secret behind that?

The secret lies in the presence of a large amount of solved questions placed in ascending order according to the beginning and end of the explanatory material.

Many studies have proven that “curiosity” is one of the most important pillars of cognitive achievement, as it is by arousing curiosity that interest and suspense can be created, and when creating suspense and challenge, the pleasure of learning increases. when student cant correctly answer  the question the first time , consequently know the correct answer and the reason for the mistake, then the knowledge is contained within the brain for a very long time.

These days, technology, especially smartphones, is used for the purpose of facilitate learning using solved questions or “questions and answers”.

This is called Education by Gamification or "learning by playing",  the idea of ​​playing is addressed because the same elements and mechanism of electronic games are used for the purposes of serious learning. 

Recently, many applications or programs that provide knowledge through playing or gamification have appeared, including KPG 1.