Tour vs. truc
While both words can translate to "trick" in English, tour usually refers to the act, performance, or prank itself, whereas truc refers to the secret mechanism, method, or gimmick used to make the trick work.
Tour
A2Examples of tour refer to a specific action, a feat of skill, a magic performance, or a prank played on someone. It focuses on the event or the execution of the deception.
Le magicien a fait un tour extraordinaire avec un lapin.
(The magician did an extraordinary trick with a rabbit.)
Je pense que mes amis m'ont joué un mauvais tour.
(I think my friends played a nasty trick on me.)
Il connait quelques tours de cartes pour impressionner les invités.
(He knows a few card tricks to impress the guests.)
Elle a plus d'un tour dans son sac.
(She has more than one trick up her sleeve.)
C'est un vilain tour du destin.
(It is a nasty trick of fate.)
Truc
B1When truc implies a trick, it specifically refers to the secret method, the knack, the hack, or the gimmick behind an illusion or success. It implies the hidden "how-to" rather than the show itself.
Je n'arrive pas à comprendre le truc derrière cette illusion.
(I cannot understand the trick behind this illusion.)
C'est un vieux truc de vendeur pour te faire payer plus cher.
(It's an old salesman trick to make you pay more.)
Le truc est de ne jamais regarder le public dans les yeux.
(The trick is to never look the audience in the eyes.)
Il y a un truc pour ouvrir cette porte sans la clé.
(There is a trick to opening this door without the key.)
Les effets spéciaux utilisent des trucs visuels complexes.
(Special effects use complex visual tricks.)
Summary
In short, use tour when describing the performance or the prank that happened (the event), and use truc when talking about the secret technique or gimmick that made it possible (the method).