Planking is a variation of an old technique said to have been developed by native Americans of the Northwest US and Canada to cook salmon. Planking enthusiasts tell us to soak a plank of wood, usually 1/4" red cedar, in water for an hour or so. Then you get a grill hot and place the plank on. Some cooks leave the plank there to pre-heat and wait for it to crackle. Food, most often salmon, is placed on the plank. By the time the food goes on, most of the water on the underside of the plank has evaporated and it begins to smoke. Very quickly the water on the sides and top evaporates, some of it slightly steaming the fish. As heat and smoke rise from below the plank, there is a low pressure area created above the food, so the smoke is pulled over the top like the air over an airplane wing. Some of the smoke lands on the food.
But not much.
Despite the propaganda, very very little smoke flavor gets on the meat, most of it only on the edges.
Some enthusiasts say planking is really a steaming method, not a smoking method since so little smoke gets onto the food. Cedar is a soft wood and absords water better than the hardwoods used for most smoking. To see how much, I took planks of 1/4" red cedar held them under water for 24 hours, much longer than the recommended one hour. On average, a 15" long plank went from 6 ounces to 7.5 ounces. That's only 1.5 ounces of water. Most of it is on the bottom of the plank, much more is on the edges, and very little is in contact with the food. If you preheat the plank as the books recommend, it doesn't take long to steam off the bottom and edges of the plank, but that steam never touches the fish. And because the fish is much colder than 212°F, it goes from 40°F in the fridge to 145°F when it is done, and because wood is a honeycomb of air and a good insulator, the little bit of water soaked into the wood beneath the fish never turns to steam.
In some ways, planking seems like a variation of cooking on a griddle or another hot surface. But on a cast iron griddle or pan, the food sears and browns where it contacts the metal because metal stores and conducts heat well, especially steel. But wood is full of air and not much of a conductor so there is no searing or deep browning of the surfaces, and as we know, brown is flavor.
There are variations on the procedure. One method calls for soaking the plank, putting it on the grill, when the bottom starts to smoke, flip it and put the food on the smoking side. I tried this. The skin of the salmon did absorb some of the carbon flavor, but much of the skin stuck to the plank, and since it did not crisp, it wasn't very appetizing. Other methods include oiling the top side, or sprinkling it with large grains of salt, or laying down a bed of herbs. Neither was more effective than salting and herbing the top side of the fish, which got more heat. Another method tells you to soak the plank in apple juice or even whiskey. Using whiskey produced a spectacular flambe when I opened the lid and air rushed it. Did a heck of a job of trimming my mustache too. Not so much flavor though. I even tried placing an aluminum pan over the plank and meat and this did create a slight smoke flavor, but nowhere nearly as much as when I put wood chips or dried herbs on the heat supply in the normal fashion.
But one of the biggest drawbacks is the cost. When you are done you must discard the expensive plank, which is badly burned on one side and full of fish juices on the other. I suppose if you are using a really thick plank you could sand both surfaces, but that's a lot more time consuming than washing a dirty griddle or wire brushing a grill grate.
Finally, there is the matter of using cedar, a soft wood. I asked the author of a book on planking why nobody burns cedar for smoking. He said "It's funny, I would never use cedar in my grill or smoker, and yet I cook food on cedar planks all the time. If I am smoking a side of salmon I choose alder or hickory."
The best feature of planking is the presentation. It looks impressive when you sit in front of your guests a slab of wood with juices running down the sides and a side of ruddy salmon astride it. But you can only do this outdoors because the bottom is usually still smoking and it will set off the smoke alarm.
And from a taste standpoint, meh. Every other method of cooking described on this site is superior.