The middle knot ballestrinque, also known as "dynamic knot", "knot U.I.A.A." or, more and more commonly, "HMS knot", is a very simple knot, commonly used by climbers as part of a lifeline or assurance system. The acronym HMS is an abbreviation of the German term Halbmastwurfsicherung, that means "assurance of half ballestrinque". The carabiners designed specifically for this technique are called HMS carabiners, although any type of carabiner can be used to give two turns to a rope inside.
In English this knot is called Munter hitch or Italian hitch. The name Munter hitch ("Munter mooring") is named after a Swiss mountain guide, Werner Munter, who popularized its use in mountaineering. The making of the knot consists basically in giving a series of turns with a cord or rope around an object, usually one round like a tube, a pole or more commonly, a carabiner. This knot allows to slide a rope through a carabiner, absorbing part of the energy and acting as a friction device. It is generally used to control the descent speed by securing a climber or as a substitute for rappelling descending apparatus. How does it work
The middle ballestrinque creates friction by rubbing the rope against itself and against the object on which it has been made. It should be noted that this knot does not cause static friction, that is: located at a point, somewhere on the string, since it is in motion and the point at which the string rubs against itself is continuously moved along their entire length. A very useful aspect of the ballestrinque medium is its reversibility: it can be pulled from either end of the rope and the knot works perfectly in either direction.
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