Solid meat (as opposed to hamburger or sausage meat) is placed into a mechanical tumbler along with a cold, seasoned liquid such as brine or crushed ice. The liquid needs to be cold to keep the meat at a safe temperature.
The Tumbler is a drum that rotates slowly, about 10 to 12 rpm. Often the tumbling is done in a vacuum to encourage absorption and to eliminate the possibility of air-borne bacteria. You put several pieces of meat in together at once; as they knock against each other and the sides of the Tumbler, abrasion loosens the protein strands in the meat, allowing fat in the muscle fibres to absorb liquid.
Depending on the meat and the product being aimed for, the meat may be tumbled for 10 to 20 minutes, then let rest for about 10 minutes, then tumbled again.
Tumbling won’t guarantee that your meat will be juicier when cooked, as some claim — water just drips and evaporates away during cooking; juiciness comes from fat. The aim of tumbling meat commercially is to add weight back into meat. In most countries, it’s regulated how much water take-up commercial meat processors can allow to happen in a piece of meat, and that the meat must be sold as “water added”, as that then makes up part of the weight. Meat processors argue that the process just replaces water lost through evaporation during some of the (admittedly very short) aging & hanging.
Besides the devious purpose of making meat heavier, Tumbling also has some legitimate purposes. It can be used to speed up the curing process by accelerating take-up of the brine. And, it is the best way to get the flavour of a marinade to go all throughout a meat. If you soak a piece of meat in a marinade for 48 hours, you are lucky if 1 to 2 % of the marinade gets into the meat. With a Tumbler, you can get up to 10% or more of the marinade into the meat, actually accomplishing something with your marinade. You not only need less marinade because more of what you do put in is taken up, but what you would marinade for 24 hours with mediocre results, you can just tumble for 20 minutes with far better results.
Home meat enthusiasts can actually buy Tumblers made for home use, with drums that detach to fit into dishwashers.