Are you looking for an easier way to untie jammed-up knots?  Most people know the standard method of looking for "ears" or "collars" of rope in the knot to shift.  What if you don't have that option, or it just isn't working?  One other method is to quickly and firmly twist the parts of the rope just outside the knot back and forth as you push in slack.

The reason this works is because of the phenomenon of compound sliding.  For an example, set a book on a slight incline on which the book will stay put.  Now, start pushing the book sideways with a pencil, and you'll notice that it'll start sliding down the ramp slowly even though it couldn't before.  Friction forces act opposite of local relative velocity, so transverse motion takes very little force to occur.  This also accounts for why it is easier to insert a plug gage in a hole if you twist as you push.

Another method, for more complex knots, requires knowledge of the way knots work.  Many knots have four parts of rope either going in or out of the knot in various places.  If you jammed a loop knot by pulling on ends 1, 2, and 3, for example, the knot may be made to change into a more workable shape by pulling hard on only ends 2 and 3, or maybe ends 3 and 4.  An easy way to exert this force is by tying the desired ends to bars, and holding down one bar with your feet while you pull the other bar with your hands. 

Because this final method could cause the knot to become more tightly jammed than before if you don't know what you're doing, this should only be used as a last resort if you can't get the twist & push method to work after several minutes.

It also helps to avoid jam-prone knots to begin with, such as the Overhand Knot, the Overhand Loop, the Figure-Eight Loop, the Flemish Bend, the Perfection Loop (Angler's Loop), the Fisherman's Knot (Water Knot/English Knot), Double Fisherman's Knot (Grapevine Knot/Double English Knot), Triple Fisherman's Knot, Scaffold Knot (any number of coils), Ring Knot, Hunter's Bend (Rigger's Bend), and others.  An ounce of prevention is worth a pound of cure.

Method of Loosening Jammed-Up Knots