"Knots weaken the rope in which they are made. When
knotted rope is strained to its breaking point, it almost always fails at the
knot or close to it, unless it is defective or damaged elsewhere."
The key is to find the very end of the string and to start to unravel it, methodically working your way along it's length and tackling each twist and turn leaving it neatly coiled and not under immense tension. Fortunately most string is readily straightened out and once the first knots and twists are loosened it all gets much easier but sometimes it is like looking at a plate of sphaghetti and you're not even sure there is only one piece of string. The knots may be fiddly and intricate and the string falls easily back into the same kinks through years of habit. Then it can be frustrating and long winded, like working in freezing temperatures with no mittens and blunt fingernails. Nevertheless the same painstaking, methodical approach works rather than attacking it with a pair of scissors and affecting a temporary and weaker repair.