Turk's head

A Turk's head was a type of knot that appeared elaborately constrictive but was in fact easily escaped. The Fourth Doctor claimed to have tied himself to Sarah with a "tangled Turk's head" on board Nerva Beacon. When she encountered resistance trying to get out of the knot, Sarah initially thought the Doctor had tied the knot incorrectly. She changed her opinion as the knot began to give way. (TV: Revenge of the Cybermen)