Thumpers - What are they used for, and how should they be designed?

Recently a lot of people have been building, and even selling, stills with "Thumpers". Unfortunately though, many of these "Thumpers" are not designed correctly, and are therefore not performing the correct function, and could potentially be dangerous.
This video seeks to explore Thumpers, how they work, what they can be used for, and what to watch out for.