Learn Polish: Umysł odpoczywa w trakcie oddechu. - The mind rests during the breath.

The phrase w trakcie (meaning "during" or "in the course of") always requires the noun that follows it to be in the Genitive case. oddechu is the Genitive form of the masculine noun oddech.

Could I use podczas instead of w trakcie in this sentence?

Absolutely. You could say Umysł odpoczywa podczas oddechu. Both podczas and w trakcie mean "during" and both require the Genitive case. w trakcie emphasizes being "in the middle of" or "in the process of" an action, but in this context, they are interchangeable.

As a masculine inanimate noun, why does the Genitive form oddechu end in -u instead of -a?

Masculine inanimate nouns are tricky in the Genitive because they can take either an -a or -u ending. While there are exceptions, abstract nouns, natural phenomena, and actions (like oddech) generally take the -u ending.

Why is the imperfective verb form odpoczywa used here?

The sentence describes a continuous state or a general, repeating truth (the mind rests while the breath is happening). Imperfective verbs like odpoczywać are used for ongoing or habitual actions, which perfectly fits the meaning of resting "during" something.