One of the things you need when you travel with your bike is to be agile when it comes to managing your gear. A bag like this makes it much easier to get things out quickly without having to reach in and rummage around.
You simply loosen the frame attachments that are attached to the seat post and saddle and pull the inner waterproof bag.
The inner bag can be compressed or extended as much as you need depending on the load, and the frame the same. It's a very compact and interesting system to travel smart :D