Putting a contract on a BRRRR?

[Tiberiu Gurau]

So I'm listening to BP Podcast 327 and Brandon gives an example of someone putting $20k down on a $100k house and him putting it under contract for $50k and investing $30k of work into it.  Why and how can you put it under contract for $50k???  I know that this is an off-the-cuff example, and the numbers may be exaggerated, but he was pretty specific about it!  

Here's the link to the podcast (example is around 24:40):

BP Podcast 327