Prussiking is the technique of climbing a rope using two or more
ascenders (also called
jammers). It is so called after the
prussik knot which allows a thin rope to be attached to a thicker one such that it can be slid up and down the thicker rope but grips it and doesn't move if weight is applied to it. Prussik knots can also be used to ascend a rope but is a lot less efficient than using modern equipment.
Since you have to lift your entire body weight prussiking can be quite tiring, especially when the pitch is 60 metres or so, such as some of the pitches in Gaping Gill.