I looked it up in Thomas Keller's book "Under Pressure." Here's what he says...
Compression is our newest sous vide technique, discovered (if you will) by Mark Hopper, chef de cuisine at Bouchon in Las Vegas. We use it lamos exclusively to change the texture of food, specifically porous fruits such as pineapple and melon. Compression, which requires a great deal of pressure, can transform a crisp, light bite of fruit into a dense, almost meaty one. We also compress cucumbers, celery, and tomatoes.
A secondary effect of compression is what might be called "setting" -- in other words, using the pressure to bring a food to a specific shape and maintain it so that when it's briefly cooked, the food sets in that shape but remains raw inside. It can then be finished using another cooking method without loosing its shape. We set the shapes of different fish and meat this way--a loing of rabbit wrapped in cacon, a piece of delicate St. Peter's fish (John Dory), a sfuffed squab breast."