Practice

Prompt to choose between timed and untimed practice

The Strengthen skills button provides a random number of  exercises reviewing material 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 one or more related skills. However, it may take one or more tries to fully strengthen a skill^{[citation needed]}.  

Best practices

Duolingo  recommends that "learners should first focus on finishing all the lessons in a skill, and later come back to review them at just the right times"

Purpose

"Strenghten skills" helps users to:

• Memorize;
• Test their knowledge;
• Review learnt material; and 
• Maintain and improve their skills.