The Garda Hitch or the Alpine Clutch...
These are two names for a hitch that is sometimes used as a one-way ratchet in a variety of systems. In other words, when the hitch is tied correctly, the rope only moves in one direction. Following is a short video on how to tie the alpine clutch:
There are some downsides to the garda hitch. First, it cannot be tied well on locking carabiners. The locks sometimes separate the carabiners just enough to make the hitch slip. Second, if the carabiners somehow lose their orientation, the hitch can slip. Third, you must use similar style carabiners to tie the knot. The problem with similar carabiners is that those that work really well, D carabiners, can unclip themselves. Fourth, the hitch is difficult to release under load. Fifth, the garda hitch can never be used instead of a belay device as it can cut the rope. And sixth, the hitch creates a lot of friction in hauling systems.
It seems a little bit sketchy to put all of your eggs into a basket that doesn't allow for locking carabiners. It also seems a little bit sketchy that if the carabiners shift, the whole thing can come apart. And there is that thing about all those other problems.... So why do people use this?
The main reason is that the garda hitch is quick. The set-up takes mere seconds.
The most common uses of the hitch are in crevasse rescue systems, pack hauling systems and in rope climbing systems. I personally tend to use the hitch in the latter two applications, which allows me to use it in "non-essential" areas or areas that are easily backed up. I don't tend to use it as the main ratchet in crevasse rescue because of the problems listed above.
Certainly many guides do use the garda very effectively in crevasse rescue systems. However, I would caution that if you do elect to use it in this way, that your system is backed up with a clove-hitch and that there is never more than six feet of slack in the system.
If this particular hitch interests you, I would suggest playing with it in your systems at non-essential points before committing to it as a main point...
--Jason D. Martin