Concept of throwaway prototype ?

Please clarify me the concept of throwaway prototype. And 2nd question is what type of risks we are facing during software prototype. 

 Prototype is a technique that can be used to give user an idea about final product at requirement stage. User views a prototype and provides us feedback. In this way, we can better judge his requirements. It is used when there is uncertainty regarding requirements. Sometimes, even the customer does not know what he/she actually needs. Important thing is that prototype is not final product. You can say that prototype is actually an earlier version of product.
Throwaway prototyping is also a prototype technique in which prototype shown to user is not made the part of final product and thrown away. It is likely to inform the final solution, but the prototype itself will not become part of the final solution. They tend to be used to answer questions. They are then discarded.
Since purpose of prototype is understanding the user’s requirements. So risks can be related to requirements as mentioned in lecture 4. Some other  risks during software prototyping can be:

1. Development progress without traditional milestones 
2. In managing conflict between developers and customers 
3. In managing the schedule for a development cycle that is essentially open-ended 
4. Processing excess change requests from customers etc.