The Bachmann Knot is actually a friction-hitch - it tightens as the load is applied. Requiring two ropes (of different diameter) and a carabiner, this knot is useful when you have a loop that needs frequent adjustments. Simply release the tension and slide the knot along the master rope using the carabiner. This knot is self-tending and can be used to ascend rope in an emergency situation (such as a crevasse rescue). See also the Prussik Knot.
Take the smaller diameter rope tie the two ends together to form a loop (using the fisherman's knot, for instance). Make as many loops around around the carabiner as you can neatly. Note that the first loop is different than the rest!
You'll want to make sure your smaller diameter rope is an appropriate length once the carabiner has been wrapped. To use, simply apply the load to the loop and the carabiner will hold fast to the guide rope. Simply release the load and you can slide the carabiner with ease.