Practice

The Strengthen skills button provides the user with a 20-exercise practice/review of material that they have learned in lessons, with a focus on the material that Duolingo believes the user most needs practice in at that moment. Such practices are untimed by default, however a timed practice option is also available for purchase in the Lingot Store.

Successfully completing a practice session results in strengthening the words practiced and, if enough words from the same skill are strengthened, also strengthening that skill.

Purpose

"Strenghten skills" helps users to:

Memorize;
Test their knowledge;
Review learnt material; and
Maintain and improve their skills (keeping their language tree refreshed/golden).