This is compared with an inductive approach, which starts with examples and asks learners to find rules, and hence is more learner-centred.
Example
The form and use of the third conditional is explained to learners, then they have a gap-fill exercise to complete, then prepare their own examples.
In the classroom
The deductive approach may be suitable with lower level learners who need a clear base from which to begin with a new language item, or with learners who are accustomed to a more traditional approach and so who lack the training to find rules themselves.
Further links:
https://www.teachingenglish.org.uk/article/presenting-new-language
https://www.teachingenglish.org.uk/article/planning-a-grammar-lesson