On the McIntyre clarinet, you'll see four pads in a row above the open hole for your left index finger. The bottom one opens when you lift your thumb for open G. 

The next pad up, for Ab, opens when you put down your left ring finger. 

The next one, for A, opens when you put down your left middle finger. If your left ring finger is not already down, this also opens the Ab pad.

The top pad, for Bb, opens when you put down both your left middle and index fingers. (It can't be operated by just your left index finger, since then you would lose the usual fingering for F#.

There's a good thread at http://www.sneezy.org/clarinet/BBoard/read.html?f=1&i=37401&t=37389 .

I met Mr. McIntyre at a show many years ago. He was a great enthusiast and played pretty well on his instrument. However, the change in fingering was big enough that you really had to stop playing for a while until you learned it thoroughly, and so it never caught on. Also, the instruments themselves were not top-line quality.

     From a private conversation with a person who had worked with the McIntyre brothers in designing and promoting this system when it first introduced in 1950s. (The McIntyre System was granted patent in 1959)

 

             "Thomas and Robert Mclntyre had a music store in Naugatuck, CT and were repairmen and engineers. They worked many years on a new configuration that would eliminate the problem of "Cross the Break"  Thomas McIntyre had made many prototypes to test. After they had come up with their best model, they applied for, and eventually were granted a patent. After their new invention was introduced, it was well received and even the famous clarinetist Bonade endorsed it. They had the clarinet bodies and keys made by a company in France (Thibouville Freres, Ivry-la-Bataille) and they assembled them in Connecticut.

           It is a hard sell because it has to use different fingerings in throat-note. But the biggest drawback to their clarinet was the weight and complexity of the new mechanism. They had solutions for it, but by this time, they were out of money and couldn't make the improvements on their own. (They funded all of this on a shoestring and through banks and mortgages) They went to a major clarinet manufacturing company (for legal reason the name will be disclosed) to try to sell their idea to them and work with their engineers on the remaining problems. It  was well received, but the company stipulated that the only way they would do it was if the Mclntyre brothers sold their patent to them. They refused the offer and Tom mentioned  that the president of the company was so angry that he told him, he would see to it personally that the clarinet would fail.

            They continued selling and marketing the clarinet on a small scale, but indeed the 'word' was spread that this was a bum invention".