Definition:
Merck Veterinary Manual (www.merckvetmanual.com) defines a hygroma as:

A hygroma is a false bursa that occurs over bony prominences and pressure points, especially in large breeds of dogs. Repeated trauma from lying on hard surfaces produces an inflammatory response, which results in a dense-walled, fluid-filled cavity. A soft, fluctuant, painless swelling develops over pressure points, especially the olecranon. If long-standing, severe inflammation may develop, and ulceration, infection, and fistulas may be present. The bursa contains a clear, yellow to red fluid.

To better understand what a hygroma is:

If you walk through your living room and "whomp" (really hit) your shin on your coffee table as many times a day as your dog goes down on its elbows, your shin is going to swell up.  Once you stop hitting your shin on the coffee table and the shin bone begins to heal the swelling will begin to go down.  The same is true with your dog's elbow, once they stop traumatizing their elbow/s, by wearing their DogLeggs, the joint will begin to heal, as it heals, the body will absorb the fluid the joint has produced (hygroma) as a response to the repeated trauma. 

This treatment is not like taking an asprin and the pain goes away in 20 minutes, rather we are trying to heal and protect the joint.

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DogLeggs recommended treatment of Hygroma:

The more your pup wears their DogLeggs, the quicker to hygroma will heal, 24 x 7 is preferable, but not always possible.

All dogs, just like people, heal at different rates. We recommend your dog wear their DogLeggs for 3 weeks after the outward signs of the hygroma are gone. We want the joint to heal from the inside out.

Your pup does not need to wear DogLeggs when out for walks, swimming etc., only when your pup will be laying down.

We recommend you remove your dog's DogLeggs once a day to inspect area of body being covered and the garment itself. If dog develops irritation, confirm placement and snugness of garment.

DogLeggs are not to be pulled up too high on the leg. DogLeggs are not a compression bandage, they are a comfortable loose fitting, protection for the joint. Once done with treatment, wash the DogLeggs and put them away. If your dog develops another hygroma, you have the treatment ready.