Shit happens in the oilfield and things get stuck downhole. Sometimes you can pull on it and jerk on with the rig, but always risk parting the tubing, pulling the derrick in on top of you and/or generally tearing something up or hurting somebody. Sometimes you can pump gooky down hole, below and up and around the stuck fish, to sort of slick everything up, then pull on it, slack off; rinse and repeat numerous times and you'll get lucky and it will come loose. Sometimes you gotta just whup on it with hydraulic jars.
Jars get run in the work string above the fishing tools. An overshot is a fishing tool that is designed to swallow the stuck fish. The overshot has grapples in it that allow you to pull up and the fish will not become "unhooked," so to speak. Hydraulic jars are run (with bumper jars, necessary if you wish to disengage from the fish), drill collars and a host of other high dollar goodies. If you pull on the fish hard enough, fluid in the jars will cause the mechanism to go off, or "hit," and helps loosen up the stuck fish. When the jars hit, it's a helluva lick downhole. Think of it as not being about to unscrew something on surface that is really tight so you wail on it with a 12 pound sledgehammer to loosen it up. "Whup on it, hand" is a common term for knocking the shit out of something to try and fix it. Sort of like jarring.
Sometimes jarring can go on and on. The tubing in the photograph above has been marked in 8 inch to 12 inch intervals so I suspect everytime the jars hit, the fish is becoming unstuck and Mike and his hands are slowly gaining on it. I've jarred on stuck stuff for days and finally the whole thing will come loose and out she all comes.
Jarring is pretty easy time for rig hands; they get to clean shit while the jars are going off, and take lots of breaks in the dog house, that sort of thing. The rig operator, above, has to stay on the floor and watch the weight indicator and pull enough over string weight with the rig to get the jars to hit. A day of this shit in 110F weather will give you a headache and along about 16:00 hrs. an operator can actually start seeing Bud Light mirages off in the distance.
Analyzing the American shale oil phenomena, or writing about it in the MSM, like Forbes, or Bloomberg, should require a license. Like a bar exam. To qualify for a license you should know what jarring is, same as knowing which end of a pipe wrench to pick up. You should also be hung upside down in a cellar and know which way to knock a hammer union up without a moment's hesitation. You should have also had to have written a check to buy working interest in an oil or gas well, at least once in your privileged life, suffered the pains and arrows of cost overruns, and had to pay lease operating expenses when there is no revenue coming in and the balance in the checkbook is zero. Nobody, I repeat NOBODY, truly understands well economics unless its from a checkbook and you are losing your monetary ass...sort of like the shale industry !
If you cannot pass those minimum requirements you should not get a license and be immediately prohibited from trying to convince people you actually know something about the oil business. You don't.