The brain dump is a an ideation technic that is helpful at the start of an ideation session.

Give to yourself or to your team about 15 minutes to write down every idea for this specific problem that

1. you already thought about in the past but didn't write down

2. you wrote down in a note somewhere

3. you find obvious 

The goal of a brain dump is to free our mind of all what we already had in our heads. By doing so, we are sure that these elements are captured and you don't get frustrated if in the other exercices the constraints make it impossible for you to express what you already had in your mind.