What is double digging a garden?

Double digging involves breaking your arms, wrists and back (at least in New England where the soil is at least 50% rocks) by digging your garden bed down 12 or more inches to loosen and improve the soil.
In our beds - which have been under lawn and hedges for the last hundred or so years - I dug through about 4-6 inches of very dusty and depleted top soil before hitting 6-10 inches of sand, clay and rock - mostly golf-ball-size gravel with 3-4 cantalope-sized rocks per 3 square feet.
I dug down about 18 inches, working in 3-4 foot sections and moving the soil I was digging out on top of the section I had just dug out and refilled. I then added back the soil I had dug out (after removing the bigger rocks) 2-3 inches at a time mixed in with 2-3 inches of soil amendments (spaghum, manure/humus and our compost), then tossed that together with a pitch fork before repeating the process of adding existing soil and soil amendments and tossing.
It takes years to build up good quality garden soil, and this is year 1. I plan to add compost and leaf mulch again in the fall and possibly even plant a cover crop for the winter.