The impromptu speech is one without any preparation. I trust that during your career the impromptu is not your most common speaking context. A few situations do occur when an impromptu speech is very important.
Most job interviews involve answering questions. Each answer might be considered an impromptu presentation.
You attend a meeting and an issue comes before the group about which you have a definite position or a vested interest. Although you were not expected to contribute, you speak to the issue.
The boss may ask you unexpectedly to give a report about your department or a recent conference.
Here is a simple formula for delivering the impromptu presentation. Begin with a positive approach. As you start, smile, look confident, and begin with an assertion. Many impromptu speeches begin with the deer-in-the-headlight look and stumbling over the first words instead of speaking with confidence. With the assertion, provide a story or example from your experiences. An assumption is that you would not offer to speak or be asked to speak unless you have experienced and/or researched the topic.
Keep the length under a couple of minutes. Otherwise, you may exhaust your knowledge and start repeating yourself or making comments that you would later regret. End by repeating a version of your opening assertion.
Most important, however, is never to deliver an impromptu speech when you are expected to deliver a prepared speech!