|MadSci Network: Physics|
Well, Gail, I'm stumped, and so are the other madsci.org volunteer and liquor store owners I asked!
The closest reference I found to your Dad's trick was an explanation of "traditional port tongs". The tongs are heated to red-hot and clamped around the neck of a bottle of port for 1-2 minutes. Then a small wet towel is applied to the spot, causing the glass to crack. (From the Into Wine website on port wine). The principles at work are those of thermal stresses and the brittle nature of glass. The hot tongs cause the glass to expand in the region where they are touched to the glass. When a wet (probably cold) towel is applied, the parts of the glass that are cooled will try to shrink rapidly. If the glass doesn't all shrink at exactly the same rate, you will generate high thermal stresses. Since glass is very brittle, once you stress the glass enough to cause a small crack, the entire circumference will fail.
I'm sorry I couldn't come up with a better response. Hopefully you will find your answer one day!
Try the links in the MadSci Library for more information on Physics.