What "rip the brief" means here is that the brief is the starting point of the conversation (i.e. just the tip of the first diamond), not the ending point.
This is in contrast to the normal client -> designer model. Where the client writes a brief that gets given to the designer and then that's the end of the discovery period and the designer is expected to get on with it and deliver something.
Bearing that in mind a good brief is one that enables the rest of the discovery phase.
It should include the things the client is trying to achieve with the design, and what constraints there are on the results. It should not include the clients visual design ideas.
Ideally it should also include any assets or design elements that the client needs included like the company logo, brand colour guidelines etc.
An example of a good paragraph from a longer brief would be something like:
"This project is aimed at 20 - 25 year olds who are looking to buy a
student railcard. We know that they are very price sensitive, so we
want to reassure them that this is a way for them to save money.
Please try to use the reuse the copy from our existing page where you can."