This is a tool I wouldn't want to do without in my kitchen. It's full name is the Jaccard Super Tendermatic Meat Tenderizer but if you refer to the "Jaccard", most food folks will know what you are talking about. Jaccard makes lots of food prep products but this is probably their most well known. There are other tenderizers on the market, but this one is better made and uses higher quality materials than most. High quality stainless steel blades in a sturdy plastic case that is machine washable. No, Jaccard is not paying me to say this and I don't intend this to be an advertisement. I'm just saying every kitchen needs a good tenderizer and this one is among the best in my opinion.
What do you do with it? Well, that's fairly obvious I suppose but I'm sure a thoughtful chef could see several good reasons to use it. First, is what I'm sure is the reason for it's invention. That is to make less expensive cuts of meat more tender. The thing about the less costly cuts such as those from the chuck or the sirloin area is that they are very flavorful but the flavor comes from the fat and connective tissues. As you would expect, the connective tissues that hold muscles together and attach muscles to bone are quite tough. In order to get the flavor out of these cuts, you need to break down the connective tissues and render out the fat. By using the tenderizer, you can perforate or start breaking the tissues down rather than relying entirely on heat to do the job. In the case of a piece of shoulder meat like a chuck steak, you would usually cook it at a lower temperature maybe with some liquid for a long time to break those connective tissues down and get all the flavor from the meat. If you have already perforated the meat, the heat can act on the connective tissues faster and a fairly tough piece of meat can be made more palatable. Perhaps even suitable for grilling. This is especially true if you have marinated the meat. I like Sirloin and serve it often and I almost always use my tenderizer and marinate the steak several hours or overnight.
That brings up the second reason for using the tenderizer. It makes all these lovely little holes for your seasoning or marinade to go into and flavor the meat through and through. If you have the capability to pull a vacuum on a container, tenderize the meat to make perforations, put it and the marinate in in the container, and apply the vaccum. This will pull the meat open and allow the marinade to not only go in the holes you've made but also down in between the muscles. I like this a lot but if you don't have a vacuum container, you are still way ahead flavor-wise to perforate the meat so the marinade can get down into the meat. I love to use this method with either beef or chicken fajitas, pork chops, and steaks.
Another reason to use a tenderizer even if you are preparing a better more tender cut, is to reduce cooking time. All those little holes let the heat get inside the meat so it cooks faster and more evenly. This is particularly helpful if you have a cut that is of uneven thickness.
I suppose the short version of the story is that you will get more tender and juicy steaks but I think it's worthhwile to spend some time thinking about how and why it works and how you might put this information to work in your kitchen. How could you use this tool in foods other than meat? Could you perforate vegetables for marinating? Something to think about...