Here’s a couple of YouTube videos of Terry using a Finis center mounted snorkel to perform drills.
http://youtu.be/XggAWXqnIOc
http://youtu.be/sdQrXBzm7yI
Here's a video on using a center mounted snorkel to perform a Side to Side drill:
http://youtu.be/yXjQoG0ysqc
The one thing that I’ve been striving to do is to keep my head still while swimming. Last summer, I had someone videotape me while swimming with a Finis swimmer snorkel. It wasn’t pretty and I was surprised at how much movement there was in my head. When I showed the video to a friend, he laughingly compared me to these guys but I was doing it underwater:
http://youtu.be/ILt5HZnPz5o