Learn Spanish (Latin America): Esta técnica es infalible, puesto que nunca falla. - This technique is infallible, since it never fails.

Puesto que means "since" or "given that." It's used to introduce a reason or cause that is often already known to the listener or is logically obvious (like an infallible technique never failing). It is a bit more formal than porque (because), which is generally used to introduce new information. Another common equivalent you'll see at this level is ya que.

Since this lesson covers subjunctive clauses, why is falla in the indicative mood here?

Conjunctions that declare a cause or an established fact, like puesto que, ya que, and porque, trigger the indicative mood because they state a reality. Here, you are affirming the fact that the technique actually never fails. You would typically only use the subjunctive with a cause if you were actively denying that cause (for example, using the structure no porque + subjunctive).

Could I use fracasar instead of fallar to say "it never fails"?

Fallar is the most natural choice here. It means to malfunction, stop working, or miss the mark, which is perfect for a technique, system, or machine. Fracasar carries a heavier, more dramatic connotation of failure, such as a business going bankrupt, a major project bombing, or a person being a failure in life.

Could I say no falla nunca instead of nunca falla?