Roger,
I have the same device as Tony. When I did the demonstration with the specialist at home that is what I was advised. I had to use the intermediate ring inside the larger ring; release the vacuum remove the intermediate ring, reinsert and create a vacuum with the larger ring.
I was also told that with my free hand which was not pressing the trigger to create the vacuum, to pull my testicles downwards to prevent them being sucked in to the tube.
If you have not had a demonstration sessions with an ED specialist, I would recommend it as he came to my home so I was able to try the device in privacy and my wife could see and feel the results http://community.prostatecanceruk.org/editors/tiny_mce/plugins/emoticons/img/smiley-laughing.gif
I hope this helps.
Davy