There are a bunch of corkscrews available to purchase and they all do the same thing. Mostly.
The task of every corkscrew is to remove a cork from a bottle of wine. But not all corks are equal. And, when it comes to older corks, they can be a bit fragile. If you've ever pulled out a prized older bottle of wine but had the cork break apart while trying to extract it, the Ah So corkscrew can be your friend.
The Ah So excels at removing fragile or brittle older corks that tend to fall apart when using a traditional corkscrew. It works by inserting the prongs between the cork and the inside of the bottle's neck. Once inserted, it pinches down on the sides of the cork. Then, by pulling up and twisting, the cork can be extracted.
Because the Ah So does not pierce the cork, it also has been called the "Butler's Friend." As the story goes, a dishonest butler could remove the cork of a bottle of wine, pour himself a bit and replace the cork without his boss knowing otherwise.
Having an Ah So on hand is not a requirement for most wine drinkers, but it can be very useful, with practice, for extracting a fragile or half-broken cork. Cheers!