In the turtlesim tutorial "Moving in a straightline", which instructions actually move the turtle?

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I am reading the turtlesim tutorial "Moving in a straightline".

http://wiki.ros.org/turtlesim/Tutoria...

I do not understand how the software moves the simulated turtle. The only relevant program segment in the tutorial seems to be in lines 8 and 37-38 (see below). But those lines are about publishing a message, vel_msg, to a channel turtle1/cmd_vel . So, I would like to know which instructions actually make the turtle move (since publishing a message does not do the job). Any clarification?

 8     velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)
...
37             #Publish the velocity
38             velocity_publisher.publish(vel_msg)