Ask the Running Doc argues that ice works because it increases circulation.
Moji argues The Technical Benefits of Icing come from reducing circulation.
I'm cool with that, clinical experience should be enough to justify the ongoing medical experiment--as it is with acupuncture, homeopathy, many allopathic medications, surgical protocols and even prayer. But remember, hypnosis works extremely well to reduce swelling and relieve pain for 10% of the population. These people have an individual proclivity to being hypnotized. If you are one of those 10% you should be using hypnosis, not ice! (Check out this book for more details.)
I suppose we could rank different healing methods for how close they adhere to the gold standard of, "I don't know how it works." Some are surely better than others? Ice has been a favorite of physical therapists probably from the beginning of the profession, even before there was clinical evidence. Physical therapists seem to be quite effective at getting people up and walking after surgery, but I have to wonder if they have ever done any broad based medical studies comparing ice, massage, and "exercises" to the old fashioned cattle prod. (If there was such a study you'd have to pay $25 on-line to read it--an industry wide standard which really does wonders for the prevalence of the "I don't know" factor.)
In the late 1980's when I first got into thinking about Chinese medicine in relationship to martial arts, ice was thought to increase the likelihood of arthritis by creating "trapped cold" in the channels. One metaphor I remember went like this: Ice is like the Highway Patrol running a traffic break in order to clear an accident (slowing qi and blood flow). On the other hand, Chinese herbs (both topical and internal) are like opening up extra lanes for traffic to go around the accident while it is being cleared up (increasing qi and blood flow).
Now-a-days most of the acupuncturists I know recommend icing at regular intervals during the first 24 hours after an injury.
I asked a muscle chemistry scientist the other day what he thought. He agreed that the science is far from settled. He thought that ice applied immediately to blunt trauma on a muscle would reduce the size of the bruise because the blood vessels which were "leaking" from damage would shrink, causing the body to lose less blood during the time it takes for clotting to take effect. However in the case of overworked muscles--muscles which are in pain from fatigue-- heat would be better. Fatigued muscles do not bruise and since the muscle cells are actually damaged but not dead, they are likely to regenerate more efficiently with heat. (The cells also grow in size after being damaged so the muscles will get bigger, and I would argue they also become more single- minded.)
I asked a professional tennis coach what he thought about ice and he explained that after 4 hours of pounding on a tennis court the legs swell so much that it is necessary to take a bath in ice up to waist! This reduces the swelling and stiffness that would otherwise make it too difficult and painful to continue training, especially over the next two days.
I suppose in thinking about this stuff we could start from the assumption that the human body is either super resilient or ultra fragile. If you are working from the super resilient angle, as I did back in the days when I was doing rosho, push-hands, and sparring three nights a week, you've probably got lots of little injuries. When I finally quit I realized how injured I was, it took about 3 months to heal at which point my practice started improving fast. On the other hand, the fragile angle is the source of all whining, and I hate whining! The fragile view of human nature is the source of all victimology and worrying. When I remember that I'm not fragile, I suddenly remember that I don't need to put up with other peoples' nonsense, like boring meetings! Down with meetings!
In offering my own experience I hope I don't sound fragile. I'm cool with weakness, but wimpy is not my thing. At one point I tried putting styrofoam cups filled with water in the freezer. I would then rip off the rim so that I had a big hunk of ice with an insulated handle I could use for ice massage. After a week of ice massaging my knee I started to feel cold channels all the way down to my foot that weren't going away from day to day. So I stopped. On the other hand, when I went to see a podiatrist he explained the hunter effect. He told me it was named after a Dr. Hunter, but I have since learned that the hunter effect got its name from actual hunters. You see hunters sometimes go hunting where it is so cold that their limbs can freeze and fall off. However, when a limb gets cold for more than about 20 minutes, the size of the hunter's blood vessels increase allowing the limb to warm on the inside, even as it is getting colder on the outside. My podiatrist claimed that ice works because when you are icing the blood vessels increase in diamiter and after about 20 minutes when you stop icing the amount of blood reaching the injury increases by as much as 4 fold.
I've heard that ice is getting used to prevent both brain and heart damage in a growing range of medical emergencies. Freeze sprays are the major technological innovation that have made Mixed Martial Arts possible. Without them there would be too much blood.
Bleeding aside, the thing most of these arguments seem to agree on is that ice reduces swelling. While not everyone agrees that swelling is bad, it is natural after all, more and more sources are coming down on the swelling-is-bad side of the argument. Prolonged swelling is thought to be really bad.
While I always recommend that if you go to an expert, you follow the doctor's instructions. The jury is still out on ice, so I also recommend that you take charge of and perform your own experiments.