Put something into the inside pocket, and close the flap. NO signal gets out!

What is a Faraday Cage?
It is a structure that blocks the passage of radio signals.

Why do you need the Faraday Cage?
Some devices can work over tens of metres, making it is easy for a (technical) crook to gain information about your devices without you knowing.
Keyfobs and electronics should remain shielded inside the faraday bag when not in use.