Youssuf always has been a fervent seeker of knowledge. He used to pester his mother and later his uncle to give him spellbooks, even though he had no wand yet. And then, after reading through the book, Youssuf asked his mother to show him the spells and the wand movement. He learned the theory of many charms and jinxes that way (once when he was irritating his uncle, he had the Full Body-Bind Curse enacted on him in practice. It was not a bad experience, actually). Youssuf wanted to become a skilled duellist ever since he was a child. To this end, he had a magical tutor as well, but he was never able, of course, to do the spells.
The truth is, Youssuf would do almost anything for a practical demonstration of some spells. And, of course, for the chance of practising them himself. And the experience of practising with other people - smart people who anticipated challenges and spells, not just dummies - would no doubt give him a taste of what a real duel would feel like. Plus, of course, he joined any activity he saw, for friends, if nothing else. Every experience adds to his arsenal of wit and intelligence. After all, as Socrates said - "The only good is knowledge and the only evil is ignorance."