Learn Russian: Запятая всегда выделяет такой оборот.

In general Russian, оборот does mean a "turn," "rotation," or "turnover." But in grammar, it specifically refers to a phrase or construction built around a certain part of speech. In this lesson's context, it refers to a деепричастный оборот (verbal adverb phrase).

What does выделяет mean in the context of punctuation?

Выделять literally means "to highlight," "to single out," or "to separate." In Russian punctuation rules, it is the standard verb for "to set off" or "to isolate" a phrase using commas. You will often see the related noun обособление (isolation) used for this grammar concept as well.

What case is такой оборот in, and why does it look like the dictionary form?

It is in the accusative case because it is the direct object being "set off" by the comma. Since оборот is an inanimate masculine noun, its accusative form is identical to its nominative form.

What makes the rule in this sentence so important for verbal adverbs?

This sentence highlights the golden rule of verbal adverb phrases (деепричастные обороты): they are always set off by commas, regardless of whether they come before or after the main verb. This is a major contrast to regular participle phrases (причастные обороты), which only take commas if they come after the noun they describe.