First things first: make sure you're familiar with your pressure cooker and whatever instructions came with it. (I've seen more than a few chefs driven to tears because they loaded their cookers without knowing how to properly affix the lid...don't let that happen to you.)
Fill your sink with cold water and tear the greens off the ribs. I usually Get about 6 pieces off each leaf. Slosh the leaves around in the water to remove dirt, bugs, etc., and tear up the big pieces as you go. You don't cut collards with a knife...'less of course you're a Yankee.
Park your pressure cooker over high heat. Grab a large mixing bowl and use it to transfer a few handfuls (about a pound) of greens to the cooker. A lot of water will cling to the greens, which is good, as it will provide a foundation for the pot liquor-literally the soul of the dish. (The bowl is just there to keep the drips off your floor.) Stir the collards for about 5 minutes or until wilted, bright green, and reduced several times in volume. Remove with tongs to another bowl, and repeat with the remaining greens. (This usually takes me 5 batches.)
Move all the greens and any accumulated liquid back to the cooker. Stir in the salt, red pepper flakes, vegetable oil, and vinegar, then nestle the ham hock down into the center of the greens. Affix the lid and allow the cooker to come to pressure. When steam is being released in a constant stream, reduce the heat to low and cook for 30 minutes, maintaining a low, even "hiss."
After half an hour, kill the heat, and release the pressure on the cooker. (Most modern cookers use a spring-loaded device or valve that will vent off the pressure.) If you don't want to wait, simply move the lidded cooker to the sink, and spray the lid with cold water until the pressure is relieved and the safety lock releases. Then, open the cooker, and use tongs to remove the greens to a bowl. Leave the hock and the liquid inside and reaffix the lid. Bring the cooker back to pressure over high heat, then reduce the heat to low and cook for another 30 minutes to thoroughly break down the hock meat.
Release the pressure and open the cooker. At this point the hock will be very broken down, and the liquid will be fragrant. Remove the hock and allow it to cool until the meat can be picked off and added to the greens.
Serve with an extra sprinkling of red pepper flakes or a few dashes of vinegar, if desired.