Well spotted...... this is taken from a Flight International flight report..
Like the Phenom 100, a ventral fin is installed on the 300, but unlike the 100 it has rudder surface. During normal operations the ventral rudder is unpowered and streamlined. In the event of failure of the primary yaw damper, which actuates the main rudder, it acts as a back-up yaw damper.
First time that i had heard of a moving ventral fin...
The rudder is split into two different units: a larger rudder unit with a conventional yaw damper set-up and, below it, the so-called ventral rudder. This fully independent rudder is not commanded by the pilot and works automatically in all phases of flight to improve dynamic stability characteristics.
Mutt