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The CEM snot is used to attach a rope to a natural anchor like a rock or tree. When you pull on the pull strand, the knot comes undone and the rope and pull strand come down together. Only a short length of rope runs over the anchor, which reduces damage to the tree or rope grooves. The CEM knot basically accomplishes the same thing as the Toggle but without the extra piece of equipment.
There are some good instruction on how to rig the CEM knot at http://ropewiki.com/CEM_knot.
The CEM knot is more prone to failure than the Toggle.
The CEM knot involves soft-on-soft rubbing, which accelerates rope damage.